This study is testing a drug called Tersolisib (LY4064809/STX-478) for advanced breast cancer with a specific genetic change known as **PIK3CA mutation**. The study aims to see how well Tersolisib works and how safe it is when used with other cancer treatments for **hormone receptor-positive (HR+)** and **human epidermal growth factor receptor 2-negative (HER2-)** breast cancer. Participants will stay in the study as long as the treatment helps without causing severe side effects. **Key Points:** - **Duration & Visits:** Participants can stay in the study as long as they benefit from it without severe side effects. The exact number of visits is not specified. - **Treatment & Compensation:** The study involves taking Tersolisib with other cancer drugs. Compensation details are not provided. - **Risks & Requirements:** Participants must follow specific contraception rules. People with certain health conditions, like diabetes or active brain metastases, cannot join the study. This study is suitable for those who have advanced breast cancer and are willing to follow study rules, including those related to contraception and medical history.