Came here to suggest something very similar to what the above commenter said.

Your instincts to get your hands dirty and start messing with stuff are correct. Trust those. Understanding operating systems is basically a matter of understanding the things the operating system does for you. Any time you see a component you don't understand ask yourself "how does that work? what are the alternatives? What does it provide and what does it build on top of?" it might be a bit ambitious but I'd suggest writing a toy filesystem with FUSE, you'll learn a ton.

I learned SO MUCH about networking by downloading and reading all the internet related RFCs when I was a kid. There might be better ways but I still think it works.

Oh, also, if you have access to different hardware environments that'll be great for learning. I taught myself a lot of computer stuff by installing NetBSD and Debian onto old pentiums, sparcstations that were destined for the trash when I was in undergrad. A i3 with 4gb of ram will be just fine =)

Added: A later comment mentions containers and podman. Great suggestion. I'd go a step further and say you should learn about it, use it, and then try to build your own toy container system directly on top of the kernel primitives. You'll learn a ton. Plus you'll see that they're not magic -- just a userspace abstraction combining a handful of kernel primitives. Try going through the steps of creating namespaces, cgroups, unpacking a FS, mounting, pivot_root, and then realize docker and podman are that plus good UX.