I learned a lot about computers by creating my own bootloader and kernel. It will touch on so much, eventually also how USB works, or SATA. I can really recomme

I learned a lot about computers by creating my own bootloader and kernel. It will touch on so much, eventually also how USB works, or SATA. I can really recommend osdev.org. It nicely combines software and learning computers! Another option is to write an emulator for a simple ISA like PICO8 or for a device like the Gameboy.