Ask HN: How do you learn big codebase fast?

Hi HN community! I just got my Facebook summer internship canceled. After some thought, I decided to spend this summer wisely by starting contributing to open-source projects. But, in my previous experience, it has been hard for me to make sense of these big projects (tried to mess up with compilers here and there). Because, there are literally hundreds of interconnected source files, and I don't know where to start from. Could you share your methods and techniques needed to learn these codebases and start contributing as fast as possible?