What’s the logic behind the software ?

Hello i'm a student , and i liked the idea of coding everything from 0 , i like to understand the logic behind stuff that are made now

Can anyone tell me where should i start to lurn how to build a software from literally 0 without any extra help of already built in function or tools that makes it easier for u , i just wanna understand how the software works and try to re create and understand how things started at first and how they're are now