I’ve been a dev for 20+ years and yeah, learning a new repo is hard. Here’s some stuff I’ve learned:

Before digging into the code:

• get the thing running and get familiar with exercising it: test happy path, edge cases, and corner cases. We’re not even looking at code yet; we’re just getting a feel for how it behaves.
• next up, see if there’s existing documentation. That’s not an end-all solution, but it’s good to see what the people that wrote the thing say about it.

Digging into the code:

• grep is your very best friend. Pick a behavior or feature you want to try and search for it in the codebase. User-facing strings and log statements are a good place to start. If you’re very lucky, you can trace it down to a line of code and search up and down from there. If you’re unlucky, they’ll take you to a localization package and you’ll have to search based on that ID.
• git blame is also your very best friend. Once you’ve got an idea where you’re working, use the blame feature on github to tie commits to PRs. This will give you a good idea of what contributing to the PR looks like, and what changes you’ll have to make for an acceptable PR.
• unit tests are also a good method of stealth documentation. You can see what different areas of the code look like in isolation, what they require, and how they behave.
• keep your own documentation file with your findings. The act of writing things down reinforces those things in your mind. They’ll be easier to recall and work with.
• if there’s an official channel for questions / support, make use of it. Try to strike a balance here: you don’t want to blow them up every five minutes, but you also don’t want to churn on a thing for days if there’s an easy answer. This is a good skill to develop in general: knowing when to ask for help, knowing when an answer will actually be helpful, and knowing when to dig for a few minutes first.

There’s no silver bullet. Just keep acquiring information until you’re comfortable.