Finally got it to build and work.... Repo here: https://github.com/mplsbugbounty/signASL-matrix/tree/master
I just have a couple of follow-up questions.

1. How could I have figured out the correct usage of buildGoModule myself? I tried searching https://search.nixos.org/packages (as well as flakes and options on the same site for good measure). Nothing came up, I presume because it's a function and not an option, flake, or package. Where is it defined in nixpkgs?
2. I got it to build with a default.nix that includes

let
  lib = import <nixpkgs> {};
in

lib.buildGoModule rec {

  pname = "signASLbot";
  version = "";

  buildInputs = [
    lib.pkg-config
...

...etc. But I notice nothing in nixpkgs does it this way, instead the whole default.nix file defines a function of an attribute set. When I tried writing my file this way I wasn't able to figure out how to call nix-build to get it to populate this attribute set correctly. Not sure exactly how to phrase this question, but how can I make my nix file more consistent with normal design patterns, and then how do I use it?