Additional Study Information: The purpose of this study is to find out whether zanidatamab combined with physicians 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 physicians 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) to or progressed (treatment stopped working) on trastuzumab deruxtecan (T-DXd). Your study doctor will check your overall health, analyze a sample of your tumor, perform scans, ask you to complete questionnaires, perform blood tests, and ask you to provide urine samples while you are in the study. Zanidatamab has been tested in several cancers including several patients with metastatic breast cancer. However, it has not yet been deemed to be safe or effective at treating your disease or any other cancers by the United States Food and Drug Administration (U.S. FDA). The other drug that is being used to compare (trastuzumab) is widely used for the type of cancer you have in combination with a chemotherapy agent (eribulin, gemcitabine, vinorelbine, or capecitabine) and has shown clinical benefit.