I cannot recommend enough Ben Eater's 6502 computer build on YouTube[1] if you want to play with a 6502 computer from the ground up. It is meant to be accessible to the ambitious amateur. It is roughly Apple 1 compatible, and he has videos on how to add new commands in Microsoft BASIC as well as how to modify Microsoft BASIC to support the computer.
Seeing him git clone Microsoft BASIC and make the changes atop it was definitely a weird but awesome moment.[2]
[1] https://youtube.com/playlist?list=PLowKtXNTBypFbtuVMUVXNR0z1...