This is a prospective, open-label, exploratory, phase II basket clinical trial designed to evaluate the efficacy and safety of sintilimab in combination with pyrotinib with or without chemotherapy in patients with advanced HER2-positive digestive system malignancies. Eligible patients include those with locally advanced unresectable or metastatic gastric, colorectal, hepatocellular, biliary tract, or pancreatic cancers. Patients will receive sintilimab and pyrotinib, with chemotherapy regimens selected at the investigator's discretion based on tumor type and clinical condition. The primary endpoint is objective response rate (ORR), with secondary endpoints including progression-free survival (PFS), disease control rate (DCR), overall survival (OS), and safety.