A Study of Novel Agents or Combinations as Perioperative Treatment in Participants With Locally Advanced Resectable Gastroesophageal Adenocarcinoma

Inclusion Criteria: * Histologically documented gastric, GEJ, or esophageal adenocarcinoma with resectable disease * Participants who are CLDN18.2-positive and HER2-negative in Sub-study 1 or HER2-positive in Sub-study 2; no specific requirements for Sub-study 3 * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Adequate organ and bone marrow function * Body weight \> 35 kg Exclusion Criteria: * Participants had any prior anti-cancer treatment or surgery for the current gastric, GEJ, or esophageal cancer. * Active or prior documented autoimmune or inflammatory disorders requiring systemic treatment with steroids or other immunosuppressive treatment * Central nervous system (CNS) pathology * Uncontrolled infections * Participants with history of (non-infectious) interstitial lung disease (ILD)/pneumonitis, current ILD/pneumonitis, or suspected ILD/pneumonitis * History of another primary malignancy * Participants with any known or suspicious distant metastasis * Uncontrolled hepatitis B and/or chronic or active hepatitis B * Current or prior use of immunosuppressive medication within 14 days before the first dose of study intervention