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?
I'm a software engineer by profession and passion and have been writing programs for well over 20 years now. I believe your experience is totally natural - at least I share the same feelings:
Large code bases take time getting to know and understand: most definitely true. It takes time and effort and is an investment you need to make before being able to feel confident. You don't need to fully comprehend every aspect of the project before you can contribute but you sure need to have a decent enough idea of how to build, test, run and deploy a particular feature. See point (2).
Don't let the size of the project intimidate you. Start small and expand your knowledge base as you go. Usually one good starting point is simply building the project, running tests and deploying it (if applicable.) Then try to take on simple tasks (eg from the project's issue tracker) and deliver on those (even things like fixing the installation docs, typos, ...) That'll have the additional impact of making you feel good about the work that you're doing and what you're learning. I'm sure at this stage you will "know" when you're confident enough to work on tasks which are a bit bigger.
During (1) and (2), please please do NOT be tempted to just blindly copy-paste stuff at the first sign of trouble. Instead invest some time and try to understand things, what is failing and why it is so. Once you do, it's totally fine to copy-paste.
After all, there's no clear cut formula. Each project is a living and breathing creature and "not one of them is like another." The only general guideline is patience, curiosity and incremental work.