Inclusion Criteria:  Advanced or metastatic HER2-expressing (IHC 2+ or 3+) or HER2-amplified solid tumors Measurable disease per the the Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1 criteria  Tumor lesion amenable for biopsy or able to submit an adequate recent archived tumor tissue for baseline testing, as follows:  Breast cancer and colorectal cancer (CRC): archival biopsy tissue obtained after the last HER2-directed therapy (excluding trastuzumab and pertuzumab), or a fresh biopsy Gastric cancer and non-small-cell lung cancer (NSCLC): archival biopsy tissue taken within the past 12 months and after completion of last HER2-directed therapy, or a fresh biopsy ECOG Performance Status of 0 or 1 Adequate hematologic, hepatic, renal, and cardiac function  Exclusion Criteria:  History of allergic reactions to certain components of study treatment therapies Untreated brain metastases Currently active (or history of) autoimmune disease Taking the equivalent of >10 mg / day of prednisone Taking a medication that moderately induces CYP2C, strongly inhibits CYP2C8, or interacts with both enzymes (CYP3A and CYP2C8) Uncontrolled or clinically significant interstitial lung disease (ILD) / pneumonitis that requires systemic corticosteroid treatment or suspected ILD / pneumonitis HIV infection, active hepatitis B or hepatitis C infection