[AnimalMuppet]

You might use a tool that would document (and help you visualize) the call graph. From that, you might get a better idea what parts are most important.

Another approach is to run it in a debugger, and just step through it, watching how it does what it does.

30K lines isn't horrible, but you shouldn't expect to understand it overnight. You should count on it taking at least a few weeks.

And, when you're done, you should leave behind a top-level architecture document, and an explanation of how it operates.