Hey there,
I have been a hobbyist programmer for quite some years and have a few smaller projects under my belt: mostly smaller GUI applications that have a few classes at maximum, make use of one or two external libraries and are very thoroughly documented and commented.
Since I love the free software movement and philosophy, I wanted to start contributing to projects I like and help them out.
The thing is, the jump from "hobbyist" to "being able to understand super-efficient compact established repos"... seems to be very hard?
Like, looking into some of these projects, I see dozens upon dozens of classes, header files, with most of them being totally oblique to me. They use syntactic constructs I cannot decipher very well because they have been optimized to irrecognizability, sometimes I cannot even find the starting point of a program properly. The code bases are decades old, use half the obscure compiler and language features, and the maintainers seem to be intimately familiar with everything to the point where I don't even know what's what or where to start. My projects were usually like four source files or so, not massive repositories with hundreds of scattered files, external configurations, edge cases, factories of factories, and so on.
If I want to change a simple thing like a placement of a button or - god knows! - introduce a new feature, I would not even remotely know where to start.
Is it just an extreme difficulty spike at this point that I have to trial-and-error through, or am I doing anything wrong?
One thing to add that I haven't seen is that for big projects, there's often nobody that could understand it all. People either get their individual components it they understand how stuff interacts, it's very rarely expected that new people in the project, even if very experienced, can just understand everything at once.
What you said that maintainers know every single fob is very frequently not the case at all! But since they get the big picture, they know in which part to look, and with their experience, they'll know what to look for in that part, it may seem to you like magic. It's not, it's just experience.
Don't get discouraged though!
Getting into big open source projects as a junior -level can be difficult, but often isn't that hard - a lot of projects often need help and will take anything they can get. And if your experience already partially aligns with what you're getting into, even better. If you reach out and be upfront about it, you'll usually get pointed in some way.
Now, you seem to only have worked on your own, with smaller code bases. That means, you don't have a problem of code organisation. So you can't understand a solution if you don't know what the problem is.
So how would you go about it?
My suggestion is to maybe get the. 10,000ft overview. Also, understand the project workflow. Projects usually have specific ways of doing things - how to build, test, run things. Try to figure out how to build and run the software on your own. If you make it, that's a great step!
Then dig into one specific component/module/part. After a bit of study, you may be able to understand that component and find a simple thing that you can change about it. If you get this far you're golden, you're doing more then a majority of users that software.
Now if you're interested, you can dig more, or reach out to devs, saying what your experience is and how far you got, and ask them if you can help. And take it from there.