I wrote a program that parses all files in the codebase (I mainly work with Objective-C frameworks and libraries), and forms a graph using all the imported 'header files' in each file. Then when I need to fix a bug or implement an API, I input the base-file name and the program spits-out all the header dependancies of the base-file in a tree format. This way I am able to limit the area of the codebase I need to focus on to solve the issue at hand. I keep on focusing on these small areas of codebase one after another and get familiarized with it pretty quickly.