Disposables are running in their own LVM volumes like any other qubes, but these volumes are created when you start the disposable and destroyed immediately when you stop the qube.
After the volumes are deleted, LVM takes care of cleaning what was stored inside How does LVM provide erased storage? but it’s not 100% guaranteed you can’t recover any of this I guess. There is a way to encrypt these volumes with temporary encryption keys that are immediately trashed, it adds some overhead but not much I guess, it does not seem documented very well though…
Someone wrote a guide explaining how to store these disposable qubes data in RAM so it’s 100% volatile Really disposable (RAM based) qubes
sys-net is not always a named disposable qube, you were prompted during Qubes OS first boot wizard after the installation if you want it to be disposable or not. It can be practical to have it not disposable when you store wifi passwords in it.