To Study the Effect of Inavolisib in Combination With Fulvestrant in Participants With Breast Cancer

Inclusion Criteria: * Documented ER +/ HER2- tumor according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines * Disease progression after or during treatment with a combination of CDK4/6i and endocrine therapy: \<= 1 prior lines of systemic therapy in the locally advanced (recurrent or progressed) or metastatic setting * Measurable or evaluable disease per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1) * Participants for whom endocrine-based therapy is recommended and treatment with cytotoxic chemotherapy is not indicated at time of entry into the study, as per national or local treatment guidelines * Confirmation of biomarker eligibility: presence of \>= 1 study-eligible PIK3CA mutation * Life expectancy of \> 6 months * Ability, in the investigator's judgment, and willingness to comply with all study -related procedures, including completion of patient-reported outcomes Exclusion Criteria: * Metaplastic breast cancer * Prior treatment with chemotherapy in the recurrent locally advanced/metastatic setting * Type 2 diabetes requiring ongoing systemic treatment at the time of study entry; or any history of Type 1 diabetes * Prior treatment with PI3K/Akt/mTOR inhibitors in the recurrent locally advanced/metastatic setting * Requirement for daily supplemental oxygen * Symptomatic active lung disease, including pneumonitis