In order to install YouTube with GNOME Software, launch GNOME Software,
search for YouTube and finally click the Install button.
a Linux terminal emulator,
type flatpak install nuvola eu.tiliado.NuvolaAppYoutube and press the Enter key to install this package.
You should be able to find YouTube in a list of installed applications. If not, you may have forgotten
to log out and log in and your desktop environment (Pantheon, GNOME, etc.) has not find out yet
that a new application has been installed.
Alternatively, you can launch YouTube from a terminal:
flatpak run eu.tiliado.NuvolaAppYoutube