example.nix

{
  inputs.nixstable.url = "github.com:NixOS/nixos-23.11";
  inputs.nixunstable.url = "github.com:NixOS/nixpkgs/unstable";
  
  outputs = { nixstable, nixunstable, ... }@inputs:
    let
      overlay = final: prev: 
      let
        overrides = [
          "pkg1"
          "pkg2"
          "pkg3"
        ];
      in (
        prev.lib.attrsets.genAttrs overrides (x: prev."${x}".override { < some override settings here > })
      );
      pkgs = import nixstable { system = "x86_64-linux"; overlays = [ overlay ]; };
      pkgsunstable = import nixunstable { system = "x86_64-linux"; overlays = [ overlay ]; };
    in {
      nixosConfigurations = ...
    };
}