The purpose of this study is to understand the effectiveness and safety of rilvegostomig in combination with chemotherapy to that of standard treatment for advanced biliary tract cancer (durvalumab in combination with gemcitabine and cisplatin). We are also performing this study to better understand advanced biliary tract cancer and associated health problems. Rilvegostomig has not been approved by the Food and Drug Administration (FDA). Durvalumab, gemcitabine, and cisplatin are FDA approved to treat biliary tract cancer, but the combination of rilvegostomig, gemcitabine, and cisplatin is...