It makes sense to invest into more thorough multi page doc, and have people/process in place to maintain it. rustc guide would be a good example: https://rustc-dev-guide.rust-lang.org/about-this-guide.html

You can take an approach like the Linux kernel where _one_ person is the sole arbiter and approver of all changes. This person is tasked with having complete, 100% depth and breadth knowledge of the entire system. It is their responsibility to ensure changes adhere to the goals of the system. It is of course an enormous bottleneck, and a big risk for management (the old 'what if they get hit by a bus, what do we do then?' concern).

Realistically, past the 200k lines of code point you aren't dealing with a codebase anymore; you're an _organization_. You need knowledge management--where do architecture decisions live, how are they approved, how are they taught to new developers, how are they updated as maintainers come and go, etc. It takes strong engineering management and leadership to keep it together.