What information sources do we use to understand a software code base.

As a developer, I've been on the receiving and giving end of the phrase "RTFM" often enough.. Many a times documentation is not accessible enough, not available, spread across a million different links and god knows when it was last updated. This can be acutely felt in a corporate environments for product based companies with multiple internal teams that creates and deprecates internal features and APIs ever so often. In this regard, documentation generation tools that use source code comments has been a great boon.This started in the form of Literate Programming as proposed by Donald Knuth in 1984!

Since documentation may be incomplete and unavailable, I was looking into alternate sources that software developers use to understand code bases and changes in them. My hypothesis is that we can utilize existing information sources like user stories,test suites etc to create an equivalent of the traditional textbook that accompanies software.

I'd like here your thoughts on what do you use (and trust) as information sources for a code base.

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edit: I had initially put this up as a survey , however the community does not allow for surveys. I am happy to discuss this topic in the thread below, and if you'd be kind enough to answer my survey, you can find the details in https://www.reddit.com/r/SampleSize/comments/js8pqw/academic_survey_on_information_sources_that_we/