The way I make myself familiar with new code is to find some process of interest and write out a psuedo code stack trace from the trigger through execution. So, function names, relevant params, leave out error handling and irrelevant branches. If a step is async, reset indentation and annotate the "stack" with "and later..."

This process can collapse large amounts of code into a hopefully a single page of text that can be well understood. Without it, I find it very difficult to keep all of the relevant code in my head when it's spread across the code base.