1. They split the code into crates that are independent of the rust of the system.
2. Start with src/main.rs or src/lib.rs to get a high level overview then go down into the things you need to understand in more detail.
3. The trick to be productive in a large codebase is thus simple: focus, one step at a time.

Tips for the documentation:

Here's advice I figured out a while ago: if you'd doing something user-facing, write the documentation. When someone asks a question, point to the place in the documentation that answers it and ask if that clarifies. If they are still confused, instead of answering them, improve the documentation, and ask them if that helped. Very quickly you get to the point where the documentation is accurate even for people who don't know the answers already.