A Study to Learn About a New Medicine Called ARV-471 (PF-07850327) in People Who Have Advanced Metastatic Breast Cancer.

The purpose of this study is to learn about the safety and effects of the study medicine ARV-471 (PF-07850327) compared to fulvestrant (FUL) in participants with advanced breast cancer. FUL is a medicine already used for treatment of breast cancer while ARV-471 is a new medicine.

This study is seeking participants with breast cancer who:

Have a disease that comes back in the place where it started or in the nearby region (locoregional recurrent disease) or have a disease that has spread to other parts of the body (metastatic) and cannot be fully cured by surgery or radiation therapy
Are responsive to hormonal therapy such as tamoxifen (it is called estrogen receptor positive disease)
Have received one line of CDK4/6 inhibitor therapy (for example palbociclib) in combination with endocrine therapy (for example letrozole) for advanced disease.
Allowed up to one additional endocrine therapy (for example exemestane) for advanced disease.

Half of the participants will be given ARV-471 while half of the participants will be given FUL.

Participants will take ARV-471 by mouth with food, one time a day. During the first treatment cycle participants will be given FUL by shots into the muscles on Day 1 and again 2 weeks later. Afterwards, FUL shots will be given on the first day of each new treatment cycle. One treatment cycle is 28 days

Participants will receive the study medicine until their breast cancer worsens or side effects become too severe. Participants will have visits at the study clinic about every 4 weeks.