I've watched an interview on MSDN with one of the developers of .NET (I think she was responsible for the GC), who also used to work on Windows, making those famous workarounds making games work on newer Windows releases, even when they relied on old kernel bugs. I think she said that the best way to get familiar with a complex new codebase is to step through it with a debugger, going through several scenarios. I think it's a great idea. I only wish I had a working visual debugger in my day to day work.

EDIT: Found the interview: <https://docs.microsoft.com/en-us/shows/Careers-Behind-the-Co...>