Installing a package is enough. For example, if you install the emacs package, Emacs.app would be available in ~/.nix-profile/Applications/Emacs.app.
If you want to launch apps you've installed with Nix using Lanchpad or Spotlight, you can use nix-darwin[1] or home-manager[2] to create a symlink in ~/Applications, though the latter seems to have temporarily disabled this feature due to conflicts between the two.
> Installing a package is enough. For example, if you install the emacs package, Emacs.app would be available in ~/.nix-profile/Applications/Emacs.app.
Right. I'm using home-manager on darwin and various apps like Emacs, Alacritty, Kitty are supported. However, many GUI apps are not, such as Firefox, so that's why I'm still using Homebrew's cask.