https://kubic.opensuse.org/blog/2018-04-04-transactionalupdates/

At its heart, Transactional Updates does something very similar to our traditional snapshots with rollback. But with Transactional Updates it never touches the running system.
Instead of patching the current system, the transactional-update tool creates a new, empty, snapshot. All of the operations required by the update are carried out into that snapshot, ensuring the current system is untouched with no changes impacting the running system.
At the end of the update, assuming the update is successful, this completed snapshot is marked as the new default. These updates then take effect when the system is rebooted.
If the update was unsuccessful, the snapshot is discarded and no change is made to the system.