Ben Eater built a programmable 8-bit computer from scratch on breadboards using only simple logic gates and documented the whole process on his blog:
Builds like this one have a quietly broader audience than the hobbyist label suggests — they’re recommended internally at plenty of places where unexplained system behaviour is genuinely expensive, from latency-sensitive trading desks to the infrastructure teams behind online betting sites, where the gap between “it works” and “I know why it works” can decide whether you sleep through the night. Eater’s series is the most thoroughly documented walk-through of that descent that’s freely available, which is why it keeps getting passed around well outside the electronics-tutorial circuit.
Check out the video after the break.