The Nixpkgs packaging guide describes how to use the Nixpkgs packaging tools in the context of contributing to Nixpkgs, where to populate the package set with your custom packages, you add them on a top-level attribute in pkgs/top-level/default.nix.

But a lot of users want to leverage Nixpkgs while instead (or simply first) adding their custom packages to an overlay or a company repo of Nix packages.