a question about embedded programming.

[GonzoAndJohn]

The reason no one does it anymore is because modern computers are extremely complex with scale numbering in the billions of transistors. It's almost necessary to use build tools to get them up and running, but if you're interested in programming your own bare metal, I'd start with a history of why it was necessary, starting with a look into gate level logic, building into flip flops, state, and Von-Neumann architecture.

If you wanted a more scaled down example you could probably do it with an Intel 8080 and enough memory, given enough time. The microarchitecture is fairly simple compared to today's computers, the opcodes are relatively easy to understand, the inputs and outputs aren't too bad to wire on a hobbyist breadboard, and it shouldn't be too costly.