And does uninstalling a flatpak app also uninstall flatpak dependencies that came with it?
from what I have seen, NO it does not do so automatically. there is a flatpak command option to clean out unused runtimes, and another to remove user data.
delete app data after uninstalling?
you either manually delete the data, or there’s some flatpak command option, or you can use a tool such as warehouse which is available as a flatpak.