A Phase II, Multicenter, Randomized, Double-Blind, Placebo-Controlled Study Evaluating the Efficacy and Safety of Inavolisib Plus Ribociclib Plus Fulvestrant Versus Placebo Plus Ribociclib Plus Fulvestrant in Patients With Endocrine- Resistant Hormone-Receptor-Positive, HER2-Negative Advanced Breast Cancer With Chromosome 8P Loss and Without a PIK3CA Mutation

Inclusion Criteria: * Women or men with histologically or cytologically confirmed carcinoma of the breast that is locally advanced or metastatic and is not amenable to surgical or radiation therapy with curative intent * Documented estrogen receptor (ER)-positive and/or progesterone receptor (PR)-positive tumor according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines, defined as \>=1% of tumor cells stained positive based on the most recent tumor biopsy and assessed locally (Allison et al. 2020) * Participants must not have received any prior systemic therapy for locally advanced unresectable or metastatic breast cancer (mBC) and must have progressed during adjuvant endocrine-based treatment or within 12 months after completing adjuvant endocrine-based therapy with an aromatase inhibitor or tamoxifen * Confirmed biomarker eligibility as documented through central laboratory testing of a tumor tissue sample documenting both the lack of a phosphatidylinositol-4,5-biphosphate 3-kinase catalytic subunit alpha gene (PIK3CA) mutation and the presence of heterozygous loss of chromosome 8p (i.e., PIK3CAnmd and chr8p loss) * Measurable disease per Response Evaluation Criteria in Solid Tumors, Version 1.1 (RECIST v1.1) Exclusion Criteria: * Metaplastic breast cancer * Radiotherapy within 2 weeks before randomization * Appropriate for treatment with cytotoxic chemotherapy at time of entry into the study, as per national or local treatment guidelines (e.g., participants with visceral crisis) * 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 Central nervous system (CNS) metastases (progressing or requiring anticonvulsants or corticosteroids for symptomatic control). Participants with a history of treated CNS metastases are eligible * Any history of leptomeningeal disease or carcinomatous meningitis