Inclusion Criteria:

          -  Is the age of majority in their country

          -  Has pathologically documented breast cancer that:

               1. Is unresectable or metastatic.

               2. Has a history of low HER2 expression, defined as IHC 2+/ISH- or IHC 1+ (ISH- or
                  untested).

               3. Is assessed by a central laboratory as low HER2 expression, defined as IHC
                  2+/ISH- or IHC 1+

               4. Is HR-positive or HR-negative. After ~60 HR-negative subjects are enrolled,
                  further enrollment will be limited to only subjects who are HR-positive (either
                  estrogen receptor positive or progesterone receptor positive per ASCO-CAP
                  guidelines).

               5. Is documented refractory to endocrine therapy, defined as having progressed on at
                  least 1 endocrine therapy and determined by the Investigator that subject would
                  no longer benefit from further treatment from endocrine therapy.

               6. If HR-positive, has or has not been treated with a CDK4/6 inhibitor. After ~240
                  HR-positive subjects have been enrolled who have not had prior therapy with a
                  CDK4/6 inhibitor, further enrollment of HR-positive subjects will be limited to
                  subjects who have had prior therapy with a CDK4/6 inhibitor.

               7. Has been treated with at least 1 and at most 2 prior lines of chemotherapy in the
                  metastatic setting. If recurrence occurred within 6 months of adjuvant
                  chemotherapy, adjuvant therapy would count as 1 line of chemotherapy.

               8. Was never previously HER2-positive (IHC 3+ or ISH+) on prior pathology testing
                  (per ASCO-CAP guidelines.)

               9. Was never previously treated with anti HER2 therapy.

          -  Has documented radiologic progression (during or after most recent treatment)

          -  Has an adequate archival tumor sample available for assessment of HER2 status by
             central laboratory (based on most recent available tumor tissue sample). If archival
             tissue is not available, a fresh biopsy is required.

          -  Has a recent tumor sample after the most recent treatment regimen or agree to undergo
             a tissue biopsy prior to randomization

          -  Has at least 1 measurable lesion based on computed tomography (CT) or magnetic
             resonance imaging (MRI), per modified Response Evaluation Criteria in Solid Tumors
             (mRECIST) version 1.1

          -  Has left ventricular ejection fraction (LVEF) ≥50%

          -  Has adequate renal function, defined as creatinine clearance ≥30 mL/min, as calculated
             using the CockcroftGault equation

          -  Has adequate hepatic function, defined as:

               1. Aspartate aminotransferase (AST)/ alanine aminotransferase (ALT) ≤5 × upper limit
                  of normal (ULN)

               2. Total bilirubin ≤1.5 × ULN) if no liver metastases or <3 × ULN in the presence of
                  documented Gilbert's syndrome (unconjugated hyperbilirubinemia) or liver
                  metastases at baseline

          -  If of reproductive/childbearing potential, agrees to follow instructions for method(s)
             of contraception

        Exclusion Criteria:

          -  Is ineligible for all 5 of the options in the physician's choice arm, either because
             of previously receiving treatment in the metastatic setting with the comparator, or
             having a contraindication to treatment

          -  Has medical history of myocardial infarction within 6 months before randomization

          -  Has history of symptomatic congestive heart failure (New York Heart Association Class
             II to IV)

          -  Has corrected QT interval (QTc) prolongation to >470 ms (females) or >450 ms (male)
             based on average of Screening triplicate 12-lead electrocardiograms (ECGs)

          -  Has a history of (noninfectious) interstitial lung disease (ILD)/pneumonitis that
             required steroids, has current ILD/pneumonitis, or where suspected ILD/pneumonitis
             cannot be ruled out by imaging at Screening

          -  Has spinal cord compression or clinically active central nervous system metastases,
             defined as untreated and symptomatic, or requiring therapy with corticosteroids or
             anticonvulsants to control associated symptoms

        NOTE: Subjects with treated brain metastases that are no longer symptomatic and who require
        no treatment with corticosteroids or anticonvulsants may be included in the study if they
        have recovered from the acute toxic effect of radiotherapy. A minimum of 2 weeks must have
        elapsed between the end of whole brain radiotherapy and study enrollment.