You can install Flatpak support using command line and install an available application from its main public repository, called Flathub.
sudo apt install flatpak
Add Flathub as a source repository.
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
You can search and install any Flatpak software. (For example Firefox).
flatpak search firefox
flatpak install flathub org.mozilla.firefox
org.mozilla.firefox is Application ID of Firefox in Flathub.
You can also install the application by downloading
On Flathub, by clicking on
Install button will download
.flatpakref file for application or you can also install application it by copying its URL. For example Firefox.
flatpak install <Path of .flatpakref file you downloaded from flathub>
flatpak install https://flathub.org/repo/appstream/org.mozilla.firefox.flatpakref