Even though you are asking general questions I'm going to assume your goal is to work on a large unfamiliar code base. Go with a call tracing tool, write one y

Even though you are asking general questions I'm going to assume your goal is to work on a large unfamiliar code base. Go with a call tracing tool, write one yourself if there is no such tool. This will speed up your learning process tremendously compared to any other approach. And in general, if learning feels hard - you are doing it wrong, try something different.