What's that thing with --extra-experimental-features?

So I tried to follow some tutorial about flakes, but it seems these are extra-experimental still.

I am using NixOS 23.11 with Nix 2.18.1 in a VM (those are the most recent stable versions, right?).

Trying around I already found out that instead of eg. nix flake update I have to use --extra-experimental-features two times to get this simple command:

nix --extra-experimental-features nix-command --extra-experimental-features flakes flake update

Searching the web I found several different things that people put into their /etc/nixos/configuration.nix to enable this globally, but none of those worked for me. I assume there is still a way to do this - can someone please tell me the correct syntax for Nix 2.18.1?

What makes things worse is that I cannot start playing around with home-manager and flakes, because home-manager switch flake . seems to use nix flake internally, which leads to errors instead of results.