More broadly, reverse engineering a codebase is a critical software activity. If you can't reverse engineer a codebase you can't debug it or add to it. Working with an existing code base is pretty much a given, unless you're developer #1 on a startup. Your success as a software developer hinges on how well you can work with an existing codebase.

One less celebrated but very valuable approach to grasp a codebase is the way you document what you've learnt. While you can learn about a codebase through exploratory debugging (mentioned by someone on a separate comment) you have to persist that knowledge on "paper" in some way. My favorite ways to do that:

  - class diagrams (for static stuff like data structures, class relationships, etc.)

  - sequence diagrams (for dynamic stuff like which functions call which and data passed between them)

With good documentation for your own learning purposes, it'll become easier to start working on a codebase and especially when you context switch, it'll help you return to a codebase more easily.