Install Flatpak apps on Ubuntu 20.04

I just installed Flatpak recently but after installing it, I can't find the apps in Ubuntu Software. I installed the Flatpak plugin for Ubuntu Software Center and I can't find any apps.

Actually, I found the flatpak sourced apps, but it signed me out of Ubuntu One, so I signed back in and then it was gone. I can't find the flatpak sourced apps.

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 

Here org.mozilla.firefox is Application ID of Firefox in Flathub.

You can also install the application by downloading .flatpakref files.

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

The pre-installed software store application in Ubuntu 20.04 is a snap application (Snap Store) (source: OMG! Ubuntu article). It doesn't seem to support the flatpak plugin.

As a workaround you can install the traditional (GNOME) Software application by running the following command in Terminal

sudo apt install gnome-software

You also need to install the gnome-software-plugin-flatpak (which you have already installed apparently).

sudo apt install gnome-software-plugin-flatpak

flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo

Additionally, you may remove the pre-installed Snap Store by

snap remove snap-store