his single-center phase II clinical study enrolls patients with unresectable locally advanced or metastatic HER2-overexpressing biliary tract cancer or urothelial carcinoma who have failed standard treatment. Participants receive trastuzumab in combination with serplulimab and chemotherapy. The study aims to evaluate the efficacy and safety of this regimen in this patient population. The primary endpoint is objective response rate (ORR). Secondary endpoints include overall survival (OS), progression-free survival (PFS), and safety.