A Study Evaluating the Efficacy and Safety of Inavolisib Plus CDK4/6 Inhibitor and Letrozole vs Placebo + CDK4/6i and Letrozole in Participants With Endocrine-Sensitive PIK3CA-Mutated, Hormone Receptor-Positive, HER2-Negative Advanced Breast Cancer

Inclusion Criteria: * Women or men with histologically or cytologically confirmed carcinoma of the breast * Documented ER-positive and/or progesterone receptor-positive tumor according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines * Documented HER2-negative tumor according to ASCO/CAP guidelines * De-novo HR+ , HER2- ABC, or, alternatively, relapsed HR+ , HER2- ABC after at least 2 years of standard neoadjuvant/adjuvant endocrine therapy without disease progression during that treatment and disease-free interval of at least 1 year since the completion of that treatment * Participants who have bilateral breast cancers which are both HR-positive and HER2-negative * Confirmation of biomarker eligibility * Consent to provide fresh or archival tumor tissue specimen * Measurable disease per Response Evaluation Criteria in Solid Tumors, Version 1.1 (RECIST v1.1) * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 * Adequate hematologic and organ function within 14 days prior to initiation of study treatment Exclusion Criteria: * Pregnant or breastfeeding, or intention of becoming pregnant during the study or within the time frame in which contraception is required * Metaplastic breast cancer * Any prior systemic therapy for locally advanced unresectable or metastatic breast cancer * Type 2 diabetes requiring ongoing systemic treatment at the time of study entry; or any history of Type 1 diabetes * Any history of leptomeningeal disease or carcinomatous meningitis * Known and untreated, or active CNS metastases. Participants with a history of treated CNS metastases are eligible * Active inflammatory or infectious conditions in either eye or history of idiopathic or autoimmune-associated uveitis in either eye * Symptomatic active lung disease * History of or active inflammatory bowel disease * Any active bowel inflammation * Prior hematopoietic stem cell or bone marrow transplantation * Treatment with strong cytochrome P450 (CYP) 3A4 inhibitors or strong CYP3A4 inducers within 4 weeks or 5 drug-elimination half-lives, prior to initiation of study treatment