I don't think that's whats being asked here - the question isn't "how do I build a PC with parts you buy on the market", rather it's "how do I manually build th

I don't think that's whats being asked here - the question isn't "how do I build a PC with parts you buy on the market", rather it's "how do I manually build the logic for a computer to understand the computations being done on an electrical level". Unless I'm very mistaken. This would involve learning about logic gates, like XOR, AND, NAND and so on, and then move up levels of abstraction until you've created, or theoretically understood, how a computer works.