But I don't want it temporarily, I want it all the time. So I want to be able to make it available for future me too, that way next time I try to use it I don't have to run `nix-shell -p <package>` again
That being said, I think there are indeed ways to partially manage your config using simple command line commands but I haven't found the need myself.
I think under the hood home-manager basically declaratively manages a Nix profile for you. You (or even better a home-manager managed .bash_profile) can then put that profile into your PATH.