So I have installed Ubuntu for someone new to Linux and need a gui tool to manage flatpak (gnome-software is broken) , I found bauh, it is quite nice. It has a unified interface to search for, install and upgrade flatpak, snap and appimages (though it only can find appimage published in Appimage-Hub, so for example Krita is not there)

Bauh is itself can be downloaded as an appimage.