If you want to probe a nix flake and figure out what it exposes, I typically just load it into nix repl and then tab complete around until I find what I'm looking for. E.g.:
$ nix repl
Welcome to Nix 2.18.5. Type :? for help.
nix-repl> :lf github:wez/wezterm/main?dir=nix
Added 12 variables.