Understanding your large codebase is a few prompts away. You can ask a model to trace through and provide reports on the project's design, architectural and implementation. From there, you can drill in with followups.

Done right, you may not know specific lines or chunks of code by heart, but much like a tuned-in company CEO, you have eyes and ears on the ground and retain global oversight and insight of the project itself. For specifics, you can learn what you need as you need it. If that means knowing how every single module works, that's just a conversation with your agent.