Another useful trick for finding where things are, is to clone (or fetch and unpack a tarball of) the nixpkgs repo. Then search that for occurrences of what you want, making sure to include a leading dot.
If you’re not already doing so, do yourself a favour and start using ripgrep. It’s in nixpkgs, so you can use Nix to install it. For instance (with a flake-enabled Nix[*])
nix shell nixpkgs#ripgrep -c rg -i '\.buildpythonpackage' path/to/your/nixpkgs/clone
shows me lots of valid, qualified, uses of buildPythonPackage, and it would do so even if I didn’t have ripgrep installed on my machine. Given that I do have ripgrep installed in my nix profile, I can omit the nix shell nixpkgs#ripgrep part, and get the same result, with just
rg -i '\.buildpythonpackage' path/to/your/nixpkgs/clone
[*] With a pre-flakes Nix, I might use
nix-shell -p --command "rg -i '\.buildpythonpack' path/to/your/nixpkgs/clone"
Edit:
I didn’t suggest searching directly in the GitHub web interface, because it gave me zero hits and made me question my sanity. This thread has both given me some peace of mind about my mental health, as well as pointing out another way of finding at you want, without the need to get a local copy of nixpkgs: https://search.nix.gsc.io/. Be sure to click on the “Advanced …” bar under the search box, and check the Ignore Case box (otherwise you’re bound to make some CamelCase error, and end up with zero hits), and then type \.builtpythonpackage in the search box.