Comment by indiestack

The unit test approach from the contractor in the thread is gold: "find a recently closed issue and try to write a unit test for it." This forces you to understand the test infrastructure, the module boundaries, and the actual behavior — not just the code structure.

I'd add one more technique that's worked well for me: trace a single request from HTTP endpoint to database and back. In a FastAPI app, that means starting at the route handler, following the dependency injection chain, seeing how the ORM/query layer works, and understanding the response serialization. You touch every layer of the stack by following one real path instead of trying to understand the whole codebase at once.

Visualizers are nice for the "big picture" but they rarely help you understand why the code works the way it does. The why is in the git history and the closed issues, not in a dependency graph.