How do you iteratively improve a broken codebase that doesn't compile with more than 3 million lines of code?

I don't want to defend the AI slop, but it's common for me to go on for a few weeks without being able to compile everything when doing something realy big. I can still compile individual modules and run their tests, but not the full application (which puts all modules together)... but it may take a lot of time until all modules can come together and actually run the app.