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.
nix-repl> outputs.packages.
outputs.packages.aarch64-darwin outputs.packages.x86_64-darwin
outputs.packages.aarch64-linux outputs.packages.x86_64-linux
nix-repl> outputs.packages.x86_64-linux.default
lf here stands for load flake. There are also a bunch of other commands in nix repl that are great to learn.
Here, I hit tab on outputs.packages. and it showed me the archs, then I hit tab once I typed the arch and I got default.
Incidentally, if you want to build it right from the repl, after loading the flake, you can do:
nix-repl> :b outputs.packages.x86_64-linux.default
This derivation produced the following outputs:
out -> /nix/store/fvdqjmr72563rkizmcg1ydbwgdlj92f5-wezterm
You can then just run that wezterm and test it out, if you want!
$ /nix/store/fvdqjmr72563rkizmcg1ydbwgdlj92f5-wezterm/bin/wezterm --version
wezterm 30345b3
Note that you can also point :lf at a flake on disk, too, if you want. E.g., if you are using a system flake:
$ nix repl
:lf /etc/nixos
You can now probe around and see what your system flake exposes! E.g., under outputs.nixosConfigurations.<node> you can inspect the configuration of the machine or manually build any derivations it exposes.