I would watch the entire thing in order to get the "full experience" of his tutorial. This touches on things like the CLI vs just syntax.
If you are coming from the perspective of non-NixOS (e.g. HomeManager or just nixpkgs) then maybe start at his rundown of what NixOS is vs HomeManager [1].
Or if you really just want to understand the nix language itself then probably start at 'Demystifying nixpkgs' [2].