PRIMARY OBJECTIVE:
      I. To determine the efficacy, based on progression-free survival (PFS), of fluorouracil,
      oxaliplatin, and leucovorin calcium (modified [m]FOLFOX6)/bevacizumab plus atezolizumab
      (combination) as compared to single agent atezolizumab.
      SECONDARY OBJECTIVES:
      I. To compare the overall survival. II. To compare the objective response rates (ORR) per
      Response Evaluation Criteria in Solid Tumors (RECIST) 1.1.
      III. To determine the safety profiles of the single agent atezolizumab and the combination of
      mFOLFOX6/bevacizumab/atezolizumab in patients with mismatch-repair deficient
      (dMMR)/microsatellite instability-high (MSI-H) metastatic colorectal cancer (mCRC).
      IV. To determine the duration of response. V. To determine the duration of stable disease.
      VI. To determine the progression-free survival (PFS) at 12 months. VII. To compare disease
      control rate (complete response [CR] + partial response [PR] + stable disease [SD]) at 12
      months.
      EXPLORATORY OBJECTIVE:
      I. To compare the health-related quality of life and patient-reported symptoms.
      TRANSLATIONAL OBJECTIVE:
      I. To bank tissue and blood samples for other future correlative studies from patients
      enrolled on the study.
      OUTLINE: Patients are randomized to 1 of 3 arms.
      ARM I: Patients receive bevacizumab intravenously (IV) over 30-90 minutes on day 1,
      oxaliplatin IV over 2 hours on day 1 of cycles 1-10, leucovorin calcium IV over 2 hours on
      day 1, and fluorouracil IV over 46-48 hours on days 1 and 2. Treatment with oxaliplatin
      repeats every 2 weeks for up to 10 cycles in the absence of disease progression or
      unacceptable toxicity. Cycles of bevacizumab, leucovorin calcium, and fluorouracil repeat
      every 2 weeks in the absence of disease progression or unacceptable toxicity. (CLOSED TO
      ACCRUAL)
      ARM II: Patients receive atezolizumab IV over 30-60 minutes on day 1. Treatment repeats every
      2 weeks for up to 48 cycles in the absence of disease progression or unacceptable toxicity.
      ARM III: Patients receive atezolizumab IV over 30-60 minutes on day 1. Treatment repeats
      every 2 weeks for up to 48 cycles in the absence of disease progression or unacceptable
      toxicity. Patients also receive bevacizumab IV over 30-90 minutes on day 1, oxaliplatin IV
      over 2 hours on day 1 cycles 1-10, leucovorin calcium IV over 2 hours on day 1, and
      fluorouracil IV over 46-48 hours on day 1. Treatment with oxaliplatin repeats every 2 weeks
      for up to 10 cycles in the absence of disease progression or unacceptable toxicity. Cycles of
      bevacizumab, leucovorin calcium, and fluorouracil repeat every 2 weeks in the absence of
      disease progression or unacceptable toxicity.
      After completion of study treatment, patients are followed up every 8 weeks for 18 months,
      and then every 12 weeks for up to 5 years.
    
        Inclusion Criteria:
          -  The patient must have signed and dated an Institutional Review Board (IRB)-approved
             consent form that conforms to federal and institutional guidelines
          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2
          -  Diagnosis of metastatic adenocarcinoma of colon or rectum without previous
             chemotherapy or any other systemic therapy for metastatic colorectal cancer
          -  Tumor determined to be mismatch-repair deficient (dMMR) by Clinical Laboratory
             Improvement Act (CLIA)-certified immunohistochemical (IHC) assay with a panel of all
             four IHC markers, including MLH1, MSH2, PMS2, and MSH6; alternatively, MSI-H diagnosed
             by polymerase chain reaction (PCR)-based assessment of microsatellite alterations
             (either Bethesda markers or Pentaplex panel) or by next-generation sequencing (NGS)
             are eligible
          -  Documentation by positron emission tomography(PET)/computed tomography (CT) scan, CT
             scan, or magnetic resonance imaging (MRI) that the patient has measurable metastatic
             disease per RECIST 1.1
          -  No immediate need for surgical intervention for the primary tumor or palliative
             diversion/bypass
          -  Absolute neutrophil count (ANC) must be >= 1500/mm^3 (obtained within 28 days prior
             randomization)
          -  Platelet count must be >= 100,000/mm^3 (obtained within 28 days prior randomization)
          -  Hemoglobin must be >= 8 g/dL (obtained within 28 days prior randomization)
          -  Total bilirubin must be =< 1.5 x ULN (upper limit of normal) for the lab unless the
             patient has a bilirubin elevation > 1.5 x ULN to 3 x ULN due to Gilbert's disease or
             similar syndrome involving slow conjugation of bilirubin (obtained within 28 days
             prior randomization); and
          -  Alkaline phosphatase must be =< 2.5 x ULN for the lab with the following exception:
             patients with documented liver metastases or bone involvement - alkaline phosphatase
             must be =< 5 x ULN (obtained within 28 days prior randomization); and
          -  Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) must be =< 3 x ULN
             for the lab with the following exception: for patients with documented liver
             metastases, AST and ALT must be =< 5 x ULN (obtained within 28 days prior
             randomization)
          -  Serum creatinine =< 1.5 x ULN for the lab or measured (24 hour urine collection) or
             calculated creatinine clearance >= 30 mL/min (obtained within 28 days prior
             randomization)
          -  A urine sample tested for proteinuria by either the dipstick method, urinalysis (UA),
             or a urine protein creatinine (UPC) ratio:
               -  The dipstick method must indicate 0-1+ protein; if dipstick reading is >= 2+, a
                  24-hour urine must be done and it must demonstrate < 1.0 g of protein per 24
                  hours
               -  A urine protein creatinine (UPC) ratio must be < 1.0; if the UPC ratio is >= 1.0
                  a 24-hour urine must be done and it must demonstrate < 1.0 g of protein per 24
                  hours
               -  Urinalysis must indicate < 30 mg/dl. If urinalysis >= 30 mg/dl, a 24-hour urine
                  must be done and it must demonstrate < 1.0 g of protein per 24 hours
          -  International normalized ratio of prothrombin time (INR) and prothrombin time (PT)
             must be =< 1.5 x ULN for the lab within 28 days before randomization; patients who are
             therapeutically treated with an agent such as warfarin may participate if they are on
             a stable dose and no underlying abnormality in coagulation parameters exists per
             medical history, regardless of PT/INR results
          -  Pregnancy test done within 14 days prior randomization must be negative (for women of
             childbearing potential only); pregnancy testing should be performed according to
             institutional standards; administration of atezolizumab or
             mFOLFOX6/bevacizumab/atezolizumab may have an adverse effect on pregnancy and poses a
             risk to the human fetus, including embryo-lethality; should a woman become pregnant or
             suspect she is pregnant while she or her partner is participating in this study, she
             should inform her treating physician immediately
          -  Women of child-bearing potential and men must agree to use adequate contraception
             methods that result in a failure rate of < 1% per year during the treatment period
             (hormonal or barrier method of birth control; abstinence) prior to study entry, for
             the duration of study participation, and for 5 months (150 days) after the last dose
             of atezolizumab, 6 months after the last dose of bevacizumab, and 6 months after the
             last dose of mFOLFOX6; a woman is considered to be of childbearing potential if she is
             not postmenopausal, has not reached a postmenopausal state (>= 12 continuous months of
             amenorrhea with no identified cause other than menopause), and has not undergone
             surgical sterilization (removal of ovaries and/or uterus); examples of contraceptive
             methods with a failure rate of < 1% per year include: bilateral tubal ligation; male
             partner sterilization; intrauterine devices; the reliability of sexual abstinence
             should be evaluated in relation to the duration of the clinical study and the
             preferred and usual lifestyle of the patient; periodic abstinence (e.g., calendar,
             ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable
             methods of contraception; men must refrain from donating sperm during this same period
        Exclusion Criteria:
          -  Patients with central nervous system (CNS) metastases are excluded, with the following
             exceptions:
               -  Patients with asymptomatic untreated CNS metastases may be enrolled, provided all
                  eligibility criteria are met, as well as the following:
                    -  Evaluable or measurable disease outside the CNS
                    -  No metastases to brain stem, midbrain, pons, medulla, cerebellum, or within
                       10 mm of the optic apparatus (optic nerves and chiasm)
                    -  No history of intracranial hemorrhage or spinal cord hemorrhage
                    -  No ongoing requirement for dexamethasone for CNS disease; patients on a
                       stable dose of anticonvulsants are permitted.
                    -  No neurosurgical resection or brain biopsy within 28 days prior to
                       randomization
               -  Patients with asymptomatic treated CNS metastases may be enrolled, provided all
                  eligibility criteria are met, as well as the following:
                    -  No radiographic demonstration and no evidence of interim progression between
                       the completion of CNS-directed therapy and the screening radiographic study
                    -  No stereotactic radiation or whole-brain radiation within 28 days prior to
                       randomization
                    -  Screening CNS radiographic study >= 28 days from completion of radiotherapy
                       and >= 14 days from discontinuation of corticosteroids
          -  Known hypersensitivity to Chinese hamster ovary cell products or other recombinant
             human antibodies, fluoropyrimidines, folic acid derivatives or oxaliplatin
          -  Uncontrolled high blood pressure defined as systolic blood pressure (BP) > 150 mmHg or
             diastolic BP 90 mmHg with or without anti-hypertensive medication; patients with
             initial BP elevations are eligible if initiation or adjustment of BP medication lowers
             pressure to meet entry criteria
          -  Any of the following cardiac conditions:
               -  Documented New York Heart Association (NYHA) class III or IV congestive heart
                  failure
               -  Myocardial infarction within 6 months prior to randomization
               -  Unstable angina within 6 months prior to randomization
               -  Symptomatic arrhythmia
          -  Serious or non-healing wound, skin ulcer, or bone fracture
          -  History of transient ischemic attack (TIA), cerebrovascular accident (CVA),
             gastrointestinal (GI) perforation or arterial thrombotic event within 6 months prior
             to randomization, symptomatic peripheral ischemia, or other medical condition in the
             opinion of the treating oncologist that makes the risk of cardiovascular or bleeding
             complications with bevacizumab use unacceptably high
          -  Other malignancies are excluded unless the patient has completed therapy for the
             malignancy >= 12 months prior to randomization and is considered disease-free;
             patients with the following cancers are eligible if diagnosed and treated within the
             past 12 months: in situ carcinomas or basal cell and squamous cell carcinoma of the
             skin
          -  Known DPD (dihydro pyrimidine dehydrogenase) deficiency
          -  Symptomatic peripheral sensory neuropathy >= grade 2 (Common Terminology Criteria for
             Adverse Events [CTCAE] version [v] 5.0)
          -  Prior treatment with oxaliplatin chemotherapy within 6 months prior to randomization
          -  Prior treatment with anti-PD-1, or anti-PD-L1 therapeutic antibody or
             pathway-targeting agents; patients who have received prior treatment with anti-CTLA-4
             may be enrolled provided the following requirements are met:
               -  Minimum of 12 weeks from the first dose of anti-CTLA-4 and > 6 weeks from the
                  last dose to randomization
               -  No history of severe immune-related adverse effects (CTCAE Grade 3 and 4) from
                  anti-CTLA-4
          -  Patients who have had chemotherapy or radiotherapy within 4 weeks (6 weeks for
             nitrosoureas or mitomycin C) prior to entering the study or those who have not
             recovered from adverse events (other than alopecia) due to agents administered more
             than 4 weeks earlier are excluded; however, the following therapies are allowed:
               -  Hormone-replacement therapy or oral contraception
               -  Herbal therapy > 7 days prior to randomization (herbal therapy intended as
                  anticancer therapy must be discontinued at least 1 week prior to randomization)
               -  Palliative radiotherapy for bone metastases > 14 days prior to randomization
          -  Treatment with systemic immunostimulatory medications (including, but not limited to
             interferon [IFN]-alpha or interleukin [IL]-2 within 42 days prior to randomization
          -  Treatment with systemic immunosuppressive medications (including, but not limited to,
             prednisone, cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor
             necrosis factor [anti-TNF] agents) within 14 days prior to randomization; however,
               -  Patients who have received acute, low dose, systemic immunosuppressant
                  medications (e.g., a one-time dose of dexamethasone for nausea; or chronic daily
                  treatment with corticosteroids with a dose of =< 10 mg/day methylprednisolone
                  equivalent) may be enrolled
               -  The use of inhaled corticosteroids and mineralocorticoids (e.g., fludrocortisone)
                  for patients with orthostatic hypotension or adrenocortical insufficiency is
                  allowed
          -  Patients taking bisphosphonate therapy for symptomatic hypercalcemia; use of
             bisphosphonate therapy for other reasons (e.g., bone metastasis or osteoporosis) is
             allowed
          -  Patients requiring treatment with a receptor activator of nuclear factor kappa-B
             ligand (RANKL) inhibitor (e.g., denosumab) who cannot discontinue it before treatment
             with atezolizumab
          -  Treatment with any other investigational agent within 4 weeks prior to randomization
          -  Known clinically significant liver disease, including active viral, alcoholic, or
             other hepatitis; cirrhosis; fatty liver; and inherited liver disease; however,
               -  Patients with past or resolved hepatitis B infection (defined as having a
                  negative hepatitis B surface antigen [HBsAg] test and a positive anti-HBc
                  [antibody to hepatitis B core antigen] antibody test) are eligible if polymerase
                  chain reaction (PCR) for HBV RNA is negative per local guidelines
               -  Patients positive for hepatitis C virus (HCV) antibody are eligible only if
                  polymerase chain reaction (PCR) is negative for HCV ribonucleic acid (RNA) per
                  local guidelines
          -  History or risk of autoimmune disease, including, but not limited to, systemic lupus
             erythematosus, rheumatoid arthritis, inflammatory bowel disease, vascular thrombosis
             associated with antiphospholipid syndrome, Wegener's granulomatosis, Sjogren's
             syndrome, Bell's palsy, Guillain-Barre syndrome, multiple sclerosis, autoimmune
             thyroid disease, vasculitis, or glomerulonephritis; however,
               -  Patients with a history of autoimmune hypothyroidism on a stable dose of thyroid
                  replacement hormone may be eligible
               -  Patients with controlled type 1 diabetes mellitus on a stable insulin regimen may
                  be eligible
               -  Patients with eczema, psoriasis, lichen simplex chronicus or vitiligo with
                  dermatologic manifestations only (e.g., patients with psoriatic arthritis would
                  be excluded) are permitted provided that they meet the following conditions:
                    -  Patients with psoriasis must have a baseline ophthalmologic exam to rule out
                       ocular manifestations
                    -  Rash must cover less than 10% of body surface area (BSA)
                    -  Disease is well controlled at baseline and only requiring low potency
                       topical steroids (e.g., hydrocortisone 2.5%, hydrocortisone butyrate 0.1%,
                       fluocinolone 0.01%, desonide 0.05%, alclometasone dipropionate 0.05%)
                    -  No acute exacerbations of underlying condition within the last 12 months
                       (not requiring psoralen plus ultraviolet A radiation [PUVA], methotrexate,
                       retinoids, biologic agents, oral calcineurin inhibitors; high potency or
                       oral steroids)
          -  History of idiopathic pulmonary fibrosis, pneumonitis (including drug induced),
             organizing pneumonia (i.e., bronchiolitis obliterans, cryptogenic organizing
             pneumonia, etc.), or evidence of active pneumonitis on screening chest computed
             tomography (CT) scan; history of radiation pneumonitis in the radiation field
             (fibrosis) is permitted
          -  History of severe allergic, anaphylactic, or other hypersensitivity reactions to
             chimeric or humanized antibodies or fusion proteins
          -  Patients with known active tuberculosis (TB) are excluded
          -  Severe infections within 28 days prior to randomization, including but not limited to,
             hospitalization for complications of infection, bacteremia, or severe pneumonia
          -  Signs or symptoms of infection within 14 days prior to randomization
          -  Received oral or intravenous (IV) antibiotics within 14 days prior to randomization;
             patients receiving prophylactic antibiotics (e.g., for prevention of a urinary tract
             infection or chronic obstructive pulmonary disease) are eligible
          -  Major surgical procedure, open biopsy, or significant traumatic injury within 28 days
             prior to randomization or anticipation of need for a major surgical procedure during
             the course of the study
          -  Administration of a live, attenuated vaccine within 28 days prior to randomization or
             anticipation that such a live, attenuated vaccine will be required during the study
             and up to 5 months after the last dose of atezolizumab; Note: influenza vaccination
             should be given during influenza season only (approximately October to March);
             patients must not receive live, attenuated influenza vaccine within 28 days prior to
             randomization or at any time during the study
          -  Psychiatric illness/social situations that would limit compliance with study
             requirements
          -  Pregnant women are excluded from this study because atezolizumab is an agent with the
             potential for teratogenic or abortifacient effects; because there is an unknown but
             potential risk for adverse events in nursing infants secondary to treatment of the
             mother with atezolizumab, breastfeeding should be discontinued if the mother is
             treated with atezolizumab; these potential risks may also apply to other agents used
             in this study; (Note: pre