What is your actual goal, beyond joining the project? Is it to upstream a fix or feature you find useful and don't want to maintain as a fork? Is it to build a personal portfolio of contributions? To learn close-up about technology the project is using? Would you eventually like someone to pay you to contribute to the project? Etc.
Find something you’re interested in and spend weeks/months reading, running, debugging the code on your own. If you’re still interested, you won’t need all the hand holding because you’ve already been working with the code. Pretend it’s like a real work situation that I’ve been in several times - there is no documentation and nobody to ask - read and run the code til you understand it.
You say that, but the surface is large with many, many corners - like a fractal.
If you just use the project for a bit you will find things, some of them might be things people will fix, others won't be fixed.
Django is something I've used for work projects for over a decade now, and there are all sorts of tickets that are older than that, just waiting to be picked up again.
Other projects of similar age and size are the same.
Don't look for a project to contribute to. Pick a real problem you have - maybe a bug you can reproduce or a feature you'd like - and fix it. If you approach a maintainer with a stackoverflow-like question "I'm trying to achieve X, I got up to point Y and am stuck on Z", it's likely they will point you in the right direction.
If you still feel like you want a specific project and you don't necessarily have a problem to solve, go into their bugzilla/issues-db and start helping moving all the bugs forward: try to reproduce them, ask users for more info, fix the tags, etc.