The purpose of this study is to find out if the combination of the drugs alpelisib and fulvestrant is better or worse than the usual approach for your type of cancer. The usual approach is defined as care most people get for endometrial cancer. Another purpose of the study is to test any good and bad effects of the drug alpelisib when used in combination with fulvestrant. Alpelisib and fulvestrant could shrink your cancer but they could also cause side effects and your cancer could grow while on treatment. Alpelisib plus fulvestrant has already been FDA-approved to treat other cancers. There will be about 25-50 people taking part in this study.