Inclusion Criteria:

          -  Part A and Part FE (M3814 + avelumab): Participants must have histologically or
             cytologically proven advanced or metastatic solid tumors for which no standard therapy
             exists, standard therapy has failed, or participants are intolerant to or have
             rejected established therapy known to provide clinical benefit for their condition

          -  Part B (M3814 + Radiotherapy [RT] + avelumab): histologically or cytologically proven
             advanced or metastatic solid tumors for which no standard therapy exists, standard
             therapy has failed, or participants are intolerant to or have rejected established
             therapy known to provide benefit for their condition and are amenable to receive RT

          -  Part A, B and FE: Measurable or evaluable disease according to Response Evaluation
             Criteria in Solid Tumors Version 1.1 (RECIST v 1.1)

          -  Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 to 1 at study
             entry

          -  Part A, B and FE: Female participants of childbearing potential should be willing to
             use a highly effective contraceptive method

          -  Part A, B and FE: Male participants should agree to refrain from donating sperm plus,
             either: abstain from any activity that allows for exposure to ejaculate

          -  Use an adequate method of contraception starting with the first dose of study therapy
             through 90 days after the last dose of study therapy

          -  Part A, B and FE: Be willing to provide informed consent for the trial

          -  Other protocol defined inclusion criteria could apply

        Exclusion Criteria:

          -  Participants who have received prior chemotherapy, hormonal anticancer therapy with
             the exception of luteinizing hormone-releasing hormone analogs, biologic therapy, or
             any other anticancer therapy within 28 days of the first dose of study treatments (6
             weeks for nitrosoureas or mitomycin C)

          -  Participants who have undergone major surgery for any reason, except diagnostic
             biopsy, within 4 weeks of the study intervention and/or has not fully recovered from
             the surgery within 4 weeks of the study intervention

          -  Participants with evidence of active or history of autoimmune disease that might
             deteriorate when receiving an immune-stimulatory agent

          -  Participants with brain metastases, except those meeting the following criteria: a)
             brain metastases that have been treated locally and are clinically stable for greater
             than or equal to (>=) 4 weeks prior to randomization b) no ongoing neurological
             symptoms that are related to the brain localization of the disease (sequelae that are
             a consequence of the treatment of the brain metastases are acceptable) c) participants
             must be either off steroids or on a stable or decreasing dose of less than (<) 10
             milligrams (mg) daily prednisone (or equivalent)

          -  Participants with severe acute or chronic medical conditions including immune colitis,
             inflammatory bowel disease, immune pneumonitis, pulmonary fibrosis or psychiatric
             conditions including recent (within the past year), psychiatric or substance abuse
             disorders; or active suicidal ideation or behavior; or laboratory abnormalities that
             may increase the risk associated with study participation or study intervention
             administration or may interfere with the interpretation of study results

          -  Participants requiring systemic immunosuppressive agents (such as steroids) for any
             reason who cannot be tapered off these drugs before start of study intervention, with
             the following exceptions: a) participants with adrenal insufficiency, may continue
             corticosteroids at physiologic replacement dose, equivalent to less than or equal to
             (<=) 10 mg prednisone daily b) participants requiring steroids through a route known
             to result in a minimal systemic exposure (topical, intranasal, intra-ocular, or
             inhalation) is permitted c) participants with previous or ongoing administration of
             systemic steroids for the management of an acute allergic phenomenon planned to be
             completed in 14 days, or that the dose after 14 days will be equivalent to <= 10 mg
             prednisone daily

          -  Participants with a history of human immunodeficiency virus (HIV) or known acquired
             immunodeficiency syndrome, Hepatitis B virus or Hepatitis C and with history of
             infection must have a polymerase chain reaction (PCR) documentation that infection is
             cleared

          -  Participants who have received a live vaccine within 30 days prior to the first dose
             of trial treatment

          -  Participants with known prior severe hypersensitivity to any of the investigational
             products or any component in its formulations

          -  Participants with evidence of additional malignancy within the last 5 years unless a
             complete remission without further recurrence was achieved at least 2 years prior to
             study entry and participants were deemed to have been cured with no additional therapy
             required or anticipated to be required. Participants with treated nonmelanoma skin
             cancers, carcinoma in situ of skin, bladder, cervix, colon/rectum, breast, or prostate
             may participate

          -  Participants pretreated with immunotherapy who have, any history of dose limiting
             toxicities (DLTs) with prior immunotherapy agents, including Grade 3/4 immune-related
             adverse events (irAEs); irreversible irAEs; Grade greater than or equals to (>=) 3
             irAEs that did not respond to steroid rescue; or neurologic irAE with significant
             clinical sequelae

          -  Participants with irAE requiring hormone replacement therapy (e.g., thyroxine,
             insulin, or physiologic dose of corticosteroid replacement therapy for adrenal or
             pituitary insufficiency) may participate as long as the endocrinopathy is well
             controlled and the participant is not otherwise symptomatic from hormone insufficiency

          -  Physiologic corticosteroid dose is defined as <= 10 mg daily of prednisone or
             equivalent

          -  for Part B only:

          -  Participants who have confirmed esophagitis and in whom radiation planning target
             volume will include any portion of the esophagus, the participant is not eligible
             unless an esophageal endoscopy rules out the presence of esophagitis

          -  Participants in whom more than 10 percent (%) of the total esophagus volume might
             receive more than 15 gray (Gy) (50% of the prescribed radiotherapy [RT] dose)

          -  Participants who have had previous radiotherapy to the same region as intended to be
             irradiated in this study within the past 12 months

          -  Participants who have had extensive previous radiotherapy on >= 30% of bone marrow
             reserve or prior bone marrow/stem cell transplantation within 5 years before study
             start

          -  If participant hepatic metastatic lesion is selected to be irradiated: - the non-tumor
             liver volume < 700 milli liters (mL); - Child-Pugh score >= 8

          -  Other protocol defined exclusion criteria could apply