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, download and install the Flathub repository file or 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