Add & remove packages by editing the /etc/nixos/configuration.nix file:
environment.systemPackages = with pkgs; [
curl
micro
nodejs
podman
python3
wget
];
Install Flatpak by setting this option:
services.flatpak.enable = true;
(For whatever reason, Flatpak can't be installed by simply listing it among other packages to install.)
Then apply the changes:
sudo nixos-rebuild switch