how to understanding something
An approach I’ve long taken to understanding something that is complex and poorly documented is the following three steps:
Read the existing documentation and the existing code, until a basic understanding is reached. Often there are serious misunderstandings or incorrect factorization at this step.
Write my own implementation, even a very basic and broken one, preferably in a completely different language (which avoids the tendency to cut and paste anything). Revise my understanding based on what works and what doesn’t.
Create new documentation and diagrams based on my new understanding. Refactor my implementation as necessary (the act of reviewing everything in order to document it often reveals incorrect factorizations). Correct documentation based on new understanding from refactoring code. Repeat until correct.