FWIK, flatpak applications can share libraries via runtimes.
If you instead mean about the host system, then yes, I think they import everything from libc and up.
I tried to use Flatpak for everything and because various Flatpak apps want their own version of Gnome I ended up with different versions of Gnome runtime installed on my system each consuming several hundred megs of disk space. At that point I decided to screw it and use Flatpak only if native repos didn't have an app I needed.