NixOS

Install Flatpak
To install Flatpak, set NixOS option services.flatpak.enable to true by putting the following into your /etc/nixos/configuration.nix:

services.flatpak.enable = true;

Then, rebuild and switch to the new configuration with:

sudo nixos-rebuild switch

For more details see the NixOS documentation.
Add the Flathub repository
Flathub is the best place to get Flatpak apps. To enable it, run:

flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
Restart
To complete setup, restart your system. Now all you have to do is install apps!

Install Flatpak