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.

From the [1]:

> 3. Add the package to the file pkgs/top-level/all-packages.nix. The Nix expression written in the first step is a function; it requires other packages in order to build it. In this step you put it all together, i.e., you call the function with the right arguments to build the actual package.

[1] https://nixos.org/manual/nix/stable/expressions/simple-expre...