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