A great way to get started is to write your own addon and get familiar with the Python API. Then as you come across bugs or missing features, you'll have a concrete goal on what you could contribute to, with a anchor to something you're doing "from the outside".
If I remember correctly, Geometry Nodes got started as a Blender addon (called "animation nodes") which eventually the Blender organization went as far as employing the creator of the addon and including it in the main release.
I'm pretty handy with the API, but the C behind it all feels pretty byzantine. I'm newish to OOP and C/C++ so before I can worry about interacting with systems like the window manager, internal database, depsgraph, renderers, RNA/DNA, etc. I gotta understand the paradigms that underpin its organization. Documentation is pretty light (but devs are very helpful in chat!).
I guess Python has just spoiled me. I need a class and "everything is right there". In Blender's C/C++, functionality always seems spread across a variety of files and callbacks in a variety of folders. I tried to follow the main loop through an extremely simple action (select a cube onscreen) to see how the stuff I know happens, happens (draw orange outline, highlight in outliner, update undo history, etc.) I knew it would be a lot, but geez, even one of those things is like a week of reading through code.
I do remember how intimidating Blender was the first time I used it though, vs how easy and natural it is now. I'm looking forward to when it will 'click' enough that I can find my way around even though I don't know how everything works. A 'core workflow' to trace through how stuff happens, if you will.
That's exactly how I've wound up contributing to a couple large open source projects. Try to build somewhat unusual stuff on top of it, find bugs/deficiencies, fix them.
If I remember correctly, Geometry Nodes got started as a Blender addon (called "animation nodes") which eventually the Blender organization went as far as employing the creator of the addon and including it in the main release.