Something I find really helpful is to start with a question that I want to answer.

Often this will be along the lines of "How does it do X?" - where X is something I either didn't know was possible or that I suspect to be really difficult.

Then I can dive in to the codebase (usually starting with GitHub code search) and try to figure out how they do it.

This helps me skip straight past the boilerplate and means I often get to a satisfying conclusion - where I've learned something new - in a very small amount of time.

And along the way I pick up knowledge about how their code is organized and often a few other tricks too.