Modern big tech software engineering is 98% supporting and maintaining.

AI does this really well, even better than new development. I regularly use Github copilot to automatically read production logs from the app and backend services, find errors logs, deeply scan the codebases, create theories about the issue, verify the theory matches production evidence, verify the theory in a sandbox env, create a failing test, and then suggest a minimal code change in a feature branch, verify test is passing now in sandbox, and submit a pr, along with a detailed analysis doc and any future design optimizations to fix the class of error.

AI is also very good at scanning legacy codebases, and reverse engineering design docs, system interfaces, and trying to reverse engineer what requirements and design tradeoffs lead to that system. It's not perfect, but already at medium software engineer level.