Seed finding takes quite a lot of number theory, and the following video is a good introduction: https://youtu.be/XVrR1WImOh8

Seed finding was originally important to find witch hut structures that are generated close by in order to make efficient farms. Soon speedrunners saw the utility of finding a good seed for their uses, and with the rise of pewdiepie's let's play many begun trying to figure out the seed for his world. Eventually this lead to the current project and packpng.com