Your question is Maintainability in Large Projects. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
What strategies would you use to ensure code maintainability in a large project?
Answer as if you are leading a team working in a growing codebase. Focus on the concrete practices, trade-offs, and rollout plan you would use to keep the system understandable, testable, and safe to change.