The purpose of this study is to find out whether zanidatamab combined with physician’s choice of chemotherapy (one of eribulin, gemcitabine, vinorelbine, or capecitabine) is safe and improves on the clinical benefit when compared to trastuzumab in combination with physician’s choice of chemotherapy (one of eribulin, gemcitabine, vinorelbine, or capecitabine) in treating the type of breast cancer you have, HER2-positive, metastatic (spread of cancer from the place where it started to other places in the body) breast cancer that is intolerant (drug is causing excessive side effects...