Interesting reading the comments how everyone has their own best way of joining a new team.

For me, the best way to join a codebase without a doubt is to actually just use the product first. What good is looking at the code if you have no idea what the product is even supposed to do?

This doesn't have to be in-depth knowledge, but just go through the setup of your product, do a few happy path use cases, feel what it's like to actually use the thing you're about to develop.