I remember having the same problem earlier in my career. I inherited a code base that was a million lines of code and 15 years old. Things I tried: printed out

I remember having the same problem earlier in my career. I inherited a code base that was a million lines of code and 15 years old. Things I tried: printed out the nastiest 2000 line function on a giant piece of paper, logged every function entry. Things that worked well at the time: reading filtered logs of actual code execution, tracing through code to see how it works. What I would do if I could talk to me 10 years ago: Don't worry about the big picture. Trace through the code path relevant to your task and make sure you understand whatever inputs/outputs there are on that path. If there's something that makes 0 sense, ask a senior dev. After tracing through enough paths, you'll start to develop a mental map, in the same way you develop a mental map of a geographical location, after visiting about five locations in the same area. You get to know the main roads, highways, etc. You have a knowledge of the relevant paths; you don't always need a map of the whole city.