How do you guys deal with working in Monolithic applications?

Think > 200 project in a single solution, each which a hefty chunk of classes.

I've been working in that codebase for almost a year now and still get lost in it.

Used to be I don't because I have the option not to. Now it's I don't because those opportunities are not available to people like me.

Check out monorepo pattern using something like nrwl or Lerna

I like to mindmap with a whiteboard. You could use a connection graphing tool like yEd. Every time you work with a section, add the classes as blocks in yEd, connect them the way they subclass in the code.
What's nice about yed is you can reorganise the tree and when you select a class it'll show the immediate dependants and dependancies up to an nth recursion. But being able to layout between family tree and organic will help with understanding the connections imo