This trial is testing zanidatamab combined with chemotherapy, and sometimes with another drug called tislelizumab, to see if it works better than the current treatment. It targets patients with advanced HER2-positive stomach and esophageal cancers that can't be treated with surgery or standard treatments. The goal is to help the immune system find and destroy cancer cells more effectively. Zanidatamab is a novel HER2-targeting agent being evaluated for its efficacy in advanced HER2-positive cancers.