This study tests the combination of two drugs, belzutifan and fulvestrant, compared to another drug, everolimus, for treating a type of breast cancer. This breast cancer is called **ER+/HER2- metastatic breast cancer**, meaning it grows because of hormones but lacks a certain protein called HER2, and it has spread to other parts of the body. The study wants to see if the new combination is safe and works well. **Belzutifan** is a newer medicine being tested for how well it stops cancer cells from growing. **Fulvestrant** is a hormone therapy that blocks the effects of estrogen, a hormone that can help cancer grow. **Everolimus** is another cancer drug that stops cancer cells from growing and dividing.

• Participants will need to provide additional tissue samples for the study.
• The study requires participants to have a good performance status, meaning they can carry out normal activities or have only slight limitations.
• Participants must have tried certain cancer treatments before, like endocrine therapy, and shown disease progression.