Also, can I “normally”/traditionally install software on NixOS, e.g. through Steam?
Depends on what you mean by traditionally. Steam works without needing any special setup by enabling it in your configuration, just programs.steam.enable = true. There’s also imperative package management with nix profile (don’t use nix-env -i which you will probably come across, it’s broken by design). Personally though I recommend sticking with the declarative configuration and nix-shell which temporarily brings packages in scope for the current shell only.