https://www.joelonsoftware.com/2000/05/26/reading-code-is-li...
Most human written code has 0 (ZERO!) docs. And if it has them, they're inaccurate or out of date or both.
Lots of code is simple and boring but a fair amount isn't and reading it is non trivial, you basically need to run it in your head or do step by step debugging in multiple scenarios.