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?
You don't, you look at the outstanding issues and pick one that sounds at all intelligible.
The experience of jumping into a big unfamiliar project is very much like Robert De Niro parachuting into the middle of hideously complex pipework to surgically fix a single leak in the movie Brazil. Nothing around you makes sense at first, and all you can do is look exactly where you've been told the problem is, and stare at it hard enough to finally see the crack in the pipe. Then as time goes on you can start checking the other pipes, and eventually you might think "Hey, why are these apartments all connected in this order? Wouldn't it be more efficient to just have a central tank that then supplies each unit?" And then maybe you try that out, and maybe it's a notable improvement and other engineers applaud you for your contribution... Or maybe it was a dumb idea for reasons that you hadn't yet grasped, in which case you get to learn why.
It can be a fun process, just give yourself time to explore. One of the best things about version control is the ability to just fuck things up in experimentation without actually damaging the project.