Setup

  services.flatpak.enable = true;

  inputs = {
    # ...
    nix-flatpak.url = "github:gmodena/nix-flatpak"; # unstable branch. Use github:gmodena/nix-flatpak/?ref=<tag> to pin releases.
  };

  outputs = { nix-flatpak, ... }: {
    nixosConfigurations.<host> = nixpkgs.lib.nixosSystem {
      modules = [
        nix-flatpak.nixosModules.nix-flatpak

        ./configuration.nix
      ];
    };
  };

Declare flatpaks

  services.flatpak.packages = [
    { appId = "com.brave.Browser"; origin = "flathub";  }
    "com.obsproject.Studio"
    "im.riot.Riot"
  ];

Updates

By default, nixos-rebuild switch will not update your flatpaks. You can have it do this with:

services.flatpak.update.onActivation = true;

Alternatively, you can use the periodic update feature which will make it so your nixos-rebuild switch command will update flatpaks if a certain amount of time has passed:

services.flatpak.update.auto = {
  enable = true;
  onCalendar = "weekly"; # Default value
};