Reading the source doesn't tell you why they chose that route or what the tradeoffs are.

Well, depends on the quality of the documentation I guess.

I agree with you, in that it's an essential skill for our job. However, from my experience at least, I've found that going head first into the source code of something you don't really understand how it works, is not the best course of action.

For example, I would make sure to read about B+ trees first, before reading the implementation of indexes in SQLite or PostgreSQL.

Agreed. A good tip for this is to try to find the earliest version/commit of the project you are interested into.

Most code bases are business logic and not nearly as complex or formal as a db. DBs, OSs and compilers are systems that should be studied to be effective in the code.

The thing about it that I find hard is knowing how to judge the important bits to read and knowing where to direct my attention as I read. Any tips on learning that?

I find that without knowing where to direct my attention, it is really hard to avoid getting distracted.