A Study to Evaluate the Efficacy and Safety of Inavolisib in Combination With Phesgo Versus Placebo in Combination With Phesgo in Participants With PIK3CA-Mutated HER2-Positive Locally Advanced or Metastatic Breast Cancer

Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1 * Histologically or cytologically confirmed and documented adenocarcinoma of the breast with metastatic or locally advanced disease not amenable to curative resection * Confirmation of HER2 biomarker eligibility based on valid results from central testing of tumor tissue documenting HER2-positivity * Confirmation of PIK3CA-mutation biomarker eligibility based on valid results from central testing of tumor tissue documenting PIK3CA-mutated tumor status * Disease-free interval from completion of adjuvant or neoadjuvant systemic non-hormonal treatment to recurrence of \>= 6 months * LVEF (left ventricular ejection fraction) of at least 50% measured by echocardiogram (ECHO) or multiple-gated acquisition scan (MUGA) * Adequate hematologic and organ function prior to initiation of study treatment Exclusion Criteria: * Prior treatment in the locally advanced or metastatic setting with any PI3K, AKT, or mTOR inhibitor or any agent whose mechanism of action is to inhibit the PI3K-AKT-mTOR pathway * Any prior systemic non-hormonal anti-cancer therapy for locally advanced or metastatic HER2-positive breast cancer prior to initiation of induction therapy * History or active inflammatory bowel disease * Disease progression within 6 months of receiving any HER2-targeted therapy * Type 2 diabetes requiring ongoing systemic treatment at the time of study entry; or any history of Type 1 diabetes * Par…