Sorry for the late reply!
I'm not sure why you think there is any 'config' involved. Perhaps you're conflating Nix the package manager with NixOS? Nix is many things, but it's also just a package manager like any other. For example:
nix profile install nixpkgs#vim # installs a package into your environment
nix shell nixpkgs#binwalk nixpkgs#vim # drops you into a shell with the given packages
nix run nixpkgs#firefox # runs the mainProgram of the given package
A couple of things to note:
The aforementioned commands are the "experimental" Nix 2.4 CLI commands that integrate with Nix Flakes. They are only experimental in name though, their status of being experimental being sort of a meme at this point. I recommend using these new commands (you can opt in by supplying a command-line flag or by tweaking the package manager config).
Packages that are not in active use (e.g. packages that you've referred to in 'nix shell' or 'nix run' invocations that are not installed using 'nix profile') are due for garbage collection. The garbage collection settings can be configured (either through NixOS, home-manager or the package manager's standalone configuration). The garbage collection can also be triggred manually. This makes experimenting with programs that you don't necessarily want to keep a joy.
The 'nixpkgs' in the aforementioned invocations is a default input that is set during installation. It refers to the nixpkgs package archive's flake's master branch's latest revision (https://github.com/nixos/nixpkgs). The available inputs are configurable (again, either through NixOS, home-manager or the package manager's standalone configuration).