Sadly this sort of thing is much easier now - thanks to the tooling. I speak from personal experience, I'd written some stub 32bit os' myself (by hand, pre-ai) and it is very fun. But you quickly run into walls that stop further development that aren't fun. I enjoyed poking at VGA registers, the 512 byte boot sector into real mode into protected mode dance. Setting up the global descriptor tables/page tables and the PIC/8042 controller.
But...then you get into protected mode you get a prompt up some very basic memory management and then TERMIOS...boring. Boring. Defining all the posix syscalls, boring. Building test harnesses ... boring.
Oh I need a DMA controller now because PIO is slow, FUN!!!
I've taken my toy os to basically Linux 2.0 level in a few weekends by throwing tokens at boring things and when I got to the end it was a little bit empty because although the kernel of the system was all me, and it very much was built on the skeleton I arranged I didn't understand bits now that while boring were important. And the magic ... was lost.
I'm working on a new version now where my rule is I hand type every line of code and I co-plan with the AI and write from my head. It's so slow going but the fun is in the problem solving and the design and by hand writing you keep enough of it in hand it feels yours and real.
Anyway this is a cool project, I hope the author enjoyed it and learned something, but this is the new normal now, powerful tooling means things that were novel are tokens away and so they lose some of the magic accordingly.