I find that the best way for me to learn a system is to just clean it up. Making non-functional changes like formatting the code and adding documentation. Maybe refactoring if I see something that could obviously be cleaned up like duplicate code.
Testing is also good, if it's testable. Based on your description, I'm guessing it isn't very testable. Testing and refactoring together is a good cycle.
Setting up a repeatable environment is huge help. Build/test should run on every commit in CI. Dockerize it if it isn't already. Have as many official environments as you see fit. At least a dev that is kept up-to-date and production, of course.
