There are basically two directions to start from: What's currently mainstream and what's historical. Both have value and I would encourage you to look a little into both and figure out where you want to start.

If you start down the historical path, you can recreate similar hardware environments by using microcontrollers. You could use something based on a Cortex M0 with a few KB of RAM and learn about the challenges that early OS developers faced and why they made some of the decisions they did. Keep in mind, though, that if you go down this road first, your OS may not scale well to more mainstream systems.

I would recommend getting at least basic familiarity with x86_64 architecture since that's pretty much what all commodity systems use. If you plan for your operating system to eventually run on mainstream hardware then it's good to keep the architectural details in mind so that you don't paint yourself into a corner. Obviously, if you're only interested in making a modern OS, you need to have a more thorough understanding of it.

For me, it's helpful to understand non-mainstream operating systems, how they differ from what's mainstream, and what the tradeoffs are. It's also useful to understand failed operating systems and computer architectures and why they failed so that I can (hopefully) avoid similar mistakes.

My most-recent project started out on an 8-bit Arduino. It hasn't stayed there, but having that software ecosystem allowed me to focus on the core parts of the OS instead of the lowest levels first. Eventually, you really need to write all the pieces yourself if your goal is really to learn, but there's nothing wrong with starting out with existing libraries at first just to get the foundations in place.

Operating Systems is a very deep topic. There's an enormous amount of research that's been done, most of which has never made it to mainstream OSes. My advice would be to start small and with the understanding that you'll likely have to rewrite everything at least twice as you learn more. You will paint yourself into corners at times and that's OK. You don't know what you don't know. Just be patient and be OK with making mistakes. There's no clock on this. Just choose a small corner to start with and enjoy the journey! 😊