If you're willing to consider a piece of advice, even the most complex rude goldberg-style codebases will yield to the program entry point. From there, you can

If you're willing to consider a piece of advice, even the most complex rude goldberg-style codebases will yield to the program entry point. From there, you can start pulling at the wool and process it one knot a time. It's tedious, slow work, but you can use note-taking, the REPL, tests (reading, if any, but also writing them), diagrams or any other tool for creating mental models until you manage it.