Flatpak lets applications share a common set of libraries, called a runtime. So org.gnome.Platform has all the libraries like glib and gtk+ that pretty much all

Flatpak lets applications share a common set of libraries, called a runtime. So org.gnome.Platform has all the libraries like glib and gtk+ that pretty much all GNOME applications use, and all that's left to build are the ones that the application bundles that aren't in the runtime.