Complete derivation for perfect play for Nim?

Question

Does anybody have a proper derivation for Nim? I don't want to see just Nim sums,binary conversions but also why we use those Nim sums and binary conversions,etc. Basically, teach it to me like i'm five. I'm a high school student who is doing this topic for my extended essay, and i dont have much knowledge of combinatorial game theory.(i can draw simple game trees and im reasonably familiar with surreal nos, im reading through On Numbers and Games right now, but again, i feel like some things are being left unexplained, such as why we have to convert hackenbush diagrams to binary code to get the values of the Games.) So the more basic you go, the better. Thank you.