This phase II study evaluates the efficacy and safety of Trastuzumab Rezetecan in combination with Adebrelimab and Lenvatinib as first-line therapy for patients with locally advanced or metastatic HER2-positive or HER2-low biliary tract cancer. The primary objective is the objective response rate (ORR). Key secondary objectives include efficacy endpoints-progression-free survival (PFS), overall survival (OS), disease control rate (DCR), and duration of response (DoR)-and safety assessments comprising adverse events (AEs), serious adverse events (SAEs), vital signs, and laboratory findings.