Follow these simple steps to start using Flatpak
A flatpak package is available in Raspberry Pi OS (previously called Raspbian) Stretch and newer. To install it, run the following as root:
# apt install flatpak
Flathub is the best place to get Flatpak apps. To enable it, run:
# flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
To complete setup, restart your system. Now all you have to do is install some apps!
Note: graphical installation of Flatpak apps may not be possible with Raspberry Pi OS.