Understanding large open-source codebases

I have started reccently to gain an interest in open-source. I have managed to make a few non-code edits on [networkx](github.com/networkx/) so I can understand the git editor procces. Now, I want to make a code edit, but when I examine the files I see quite a few files with no idea of how they relate to each other. How do I examine the codebase and step through the files with a debugger