Rilvegostomig is being tested in this study to see if it helps people with BTC (Biliary Tract Cancer) after surgery. The study will compare this new drug with a placebo (a "fake" treatment) alongside chemotherapy. Chemotherapy uses strong medicine to kill cancer cells, and the types used in this study include capecitabine, S-1, or gemcitabine/cisplatin. The goal is to find out if rilvegostomig can help prevent the cancer from coming back. The study is for patients who have had a complete surgery to remove the cancer and are currently cancer-free. It’s a Phase III study, which means the drug has already been tested for safety and is now being tested for how well it works.

• The study is conducted globally and it's a double-blind study, meaning neither doctors nor patients know who gets the real drug or placebo.
• Participants need to have had surgery recently (within 12 weeks) and be free of active disease.
• Patients can't have certain other health problems or have taken certain medications recently.