It involves a lot of tracing where something happens, just following code, or using logging and debuggers. Once I have found the place I need to be it's usually pretty easy to make minor changes or add similar functionality. Along the way I pick up a lot of info on how things work and where they are located. Over time that adds up.
For new languages/frameworks/libraries docs can be good for the basics and to fill in the gaps.
I generally learn best when trying to get something done, I recognize that might be different for others. A goal helps me focus; instead of learning everything at once, I learn only what I need to achieve my goal, and achieving goals motivates me to keep going.