packages listed in configuration.nix get added to a system profile. This also gives you the generations
nix-env is meant to give you a similar experience to other package managers, however, i manages it’s own gcroots (what derivations can be cleaned up), and even generations. Generally you should avoid using nix-env for package installation as it doesn’t leverage the power of nix.
$ echo $PATH
/home/jon//.cargo/bin:/home/jon//.config was added by me.