There are many different types of jobs. I will assume you mean low level development on an otherwise normal operating system like Linux.
I would start by learning how Linux userspace is constructed. Dig up old Linux From Scratch docs, build your minimal system, experiment with it. Try to understand how different stuff is put together. How elf/static/dynamic libraries works, etc. Look at your running processes (there shouldn't be many if you do LFS) and try to explain everything you see (what this process does, how it does its job, etc.)
Learn how kernel communicates with userspace. What are devices? What are syscalls? What syscalls are available? How filesystems work? How devices work? Etc.
Learn what is the job of Linux kernel. How memory management works, what is virtual memory, what is the difference when programming in user/kernel space.
The best way to learn system programming is definitely not getting up to your ears in a single open source project. You need variety of knowledge because you want to understand in breadth how the system works. Once you get a job or you figure out something interests you more there will be good time to specialize (say you want to specialize in security or containerization etc.)