建成 is a resultative verb (r.v.) that combines 建 'to build' with 成 'to complete/completed'. Unlike 建 or 建设, which focus on the process of building, 建成 marks the successful completion of construction — it means 'to finish building' or 'to be built/completed'. It is commonly used for buildings, bridges, systems, or infrastructure that have reached completion. You can think of it as 'build + become done'.