A Study Evaluating the Efficacy and Safety of Inavolisib + Palbociclib + Fulvestrant vs Placebo + Palbociclib + Fulvestrant in Participants With PIK3CA-Mutant, Hormone Receptor-Positive, HER2-Negative, Locally Advanced or Metastatic Breast Cancer

Inclusion Criteria * Confirmed diagnosis of HR+/HER2- breast cancer * Metastatic or locally advanced disease not amenable to curative therapy * Progression of disease during adjuvant endocrine treatment or within 12 months of completing adjuvant endocrine therapy with an aromatase inhibitor or tamoxifen * Receiving LHRH agonist therapy for at least 2 weeks prior to Day 1 of Cycle 1 if pre/peri-menopausal * Confirmation of biomarker eligibility (detection of specified mutation(s) of PIK3CA via specified test) * Consent to provide fresh or archival tumor tissue specimen * Measurable disease per Response Evaluation Criteria in Solid Tumors, Version 1.1 (RECIST v1.1); evaluable "bone-only" disease is not eligible; "bone-only" disease with at least one measurable, soft-tissue component, even if considered disease that is limited to bone but has lytic or mixed lytic/blastic lesions and at least one measurable soft-tissue component per RECIST v1.1 may be eligible * Eastern Cooperative Oncology Group Performance Status of 0 or 1 * Life expectancy of \> 6 months * Adequate hematologic and organ function within 14 days prior to initiation of study treatment Exclusion Criteria * Metaplastic breast cancer * Any history of leptomeningeal disease or carcinomatous meningitis * Any prior systemic therapy for metastatic breast cancer * Prior treatment with fulvestrant or any selective estrogen-receptor degrader, with the exception of participants that have received fulvestrant or any selective estrogen-receptor degrader as part of neoadjuvant therapy only and with treatment duration of no longer than 6 months * Prior treatment with any PI3K, AKT, or mTOR inhibitor, or any agent whose mechanism of action is to inhibit the PI3K-AKT-mTOR pathway * Type 2 diabetes requiring ongoing systemic treatment at the time of study entry; or any history of Type 1 diabetes * Known and untreated, or active CNS metastases. Patients with a history of treated CNS metastases may be eligible * Active inflammatory or infectious conditions in either eye, or any eye conditions expected to require surgery during the study treatment period * Symptomatic active lung disease, or requiring daily supplemental oxygen * History of inflammatory bowel disease or active bowel inflammation * Anti-cancer therapy within 2 weeks before study entry * Investigational drug(s) within 4 weeks before randomization * Prior radiotherapy to \>= 25% of bone marrow, or hematopoietic stem cell or bone marrow transplantation * Chronic corticosteroid therapy or immunosuppressants * Pregnant, lactating, or breastfeeding, or intending to become pregnant during the study or within 2 weeks after the final dose of study treatment * Major surgical procedure, or significant traumatic injury, within 28 days prior to Day 1 of Cycle 1