I mean, it can be, because there are 3 different ways on NixOS:
configuration.nix, to have it installed for all isers and have it saved in the configuration so you can reproduce it.
nix-env, which installs it locally for the current user and doesn’t save in the configuration.nix
nix-shell, to install this package in a temporary shell, so the package is never really installed on your system, it’s just there temporarily, until you exit that shell session.
I mean, it can be, because there are 3 different ways on NixOS:
configuration.nix, to have it installed for all isers and have it saved in the configuration so you can reproduce it.
nix-env, which installs it locally for the current user and doesn’t save in the configuration.nix
nix-shell, to install this package in a temporary shell, so the package is never really installed on your system, it’s just there temporarily, until you exit that shell session.