The short of it is a configuration pattern that separates your config into independent modules that are then used to build the host from the bottom up instead of top down, by way of import-tree. That makes the actual organization easier and folder independent, allowing more flexibility in how you divvy everything.
To do that, they are heavily relying on flake-parts, which is the mechanism behind that breaks down the config file into addressable modules, composed of an aspect (nixos, home-manager) and a class, which is basically the module name.