Trastuzumab is an anticancer medication that targets HER2, a protein overproduced by some cancers of the stomach (gastric cancers) and the junction between the stomach and esophagus (gastro-esophageal junction or GEJ cancers). In this study, researchers are evaluating the safety of combining the drug trastuzumab deruxtecan (T-DXd) with immunotherapy and/or chemotherapy in patients with inoperable or metastatic stomach and GEJ cancers that have come back or continued to grow despite prior treatment.