Here's the secret to grokking a software project: a given codebase is not understandable without understanding how and why it was built; i.e. if you didn't build it, you're not going to understand why it is the way it is.

In theory, the codebase should be, as it is, understandable (and it is, with a great deal of rigorous study). In reality, that's simply not the case, not for any non-trivial software system.