The pre-installed apps are on a read-only partition. And it's not a stub of the app, it is the full app. When you install an update for one of those apps, the d

The pre-installed apps are on a read-only partition. And it's not a stub of the app, it is the full app. When you install an update for one of those apps, the device will have both the original version on the read-only partition and the updated version on a mutable one. Uninstalling will only remove the updated version. There is no explicit factory image stored somewhere, the reset to a factory state is achieved by clearing the mutable storage. This achieves getting the device to a clean state since the read-only storage is guaranteed to be in the same state as to begin with. (The read-only storage does get updated on an OS upgrade. That's why a factory reset will not downgrade the OS itself.)