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

* 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