Your question is Debugging an Undocumented Legacy System. 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).
Tell me about a time you had to debug a complex issue in a legacy codebase with little or no documentation. How did you create clarity, decide where to focus, and make sure the team was better positioned after the issue was resolved?