There's no way around just learning the codebase. I have never seen code documentation that was complete or correct, let alone both.
But the documentation can really help in telling why we are doing things. That also seeps in to naming things like classes. If that were not so, we'd just name everything Class1, Class2, Method1, Method2 and so on.
I have written code that was correct and necessarily written the way it was oly to have it repeatedly altered by well meaning colleagues who thought it looked wrong, inefficient, or unidiomatic. Eventually I had to fill it with warning comments and write a substantial essay explaining why it had to be the way it was,
Code tells you what is happening but it doesn't always do it so that it is easy to understand and it almost never tells you why something is the way it is.