packages
you can use the nix profile commands to manage packages for yourself, without needing any extra permissions.
use search.nixos.org to search for packages.
here's a a short overview of some commands you might need:
install a package
$ nix profile add nixpkgs#fastfetch
remove a package
$ nix profile remove fastfetch
list all installed packages
$ nix profile list
temporarily "install" a package
$ nix shell nixpkgs#fastfetch
the package will only be usable in the current shell.