RO7771950 Versus Tucatinib in Combination With Trastuzumab and Capecitabine in People With Locally Advanced or Metastatic Breast Cancer That is Human Epidermal Growth Factor Receptor 2 (HER2)-Positive

Inclusion Criteria: * Pathologically documented locally advanced inoperable (LAI) or metastatic breast cancer (MBC) with confirmed HER2-positive status by central laboratory * Measurable disease only as per by RECIST v1.1/RANO-BM in stage 1. Non-measurable disease allowed in stage 2. * Previously treated (stable or progressive) or previously untreated CNS metastases, or leptomeningeal metastases * At least one prior line of anti-HER2-based therapy for LAI or metastatic disease * Prior anti-HER2 antibody-drug conjugate (ADC), such as trastuzumab-deruxtecan (T-DXd) or trastuzumab emtansine (T-DM1), in any treatment setting. Participants for whom prior ADC therapy was not appropriate (e.g., due to lack of access or being medically unfit) may be considered for enrollment. * Prior tyrosine kinase inhibitor (TKI) in the (neo)adjuvant setting provided completion is \> 12 months ahead of LAI occurrence. Prior treatment with TKIs for LAI/MBC is not permitted. * Has protocol-defined adequate organ and bone marrow function * Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 * Baseline left ventricular ejection fraction (LVEF) \>/= 50% Exclusion Criteria: * Concurrent anti-cancer treatment, or treatment with investigational therapy within 28 days prior to initiation of study treatment * Known active/untreated hepatitis B or C or chronic liver disease * Clinically significant cardiovascular disease or risk, including heart failure (New York Heart Association (NYHA) ≥ …