Complex sentence: A truck for another Sonderkommando unit then arrives on the scene, and Saul sneaks on to it, heading to a nearby riverbank, where the prisoners are ordered to put the ashes from the crematoriums into the river.
Split into simpler sentences: A truck for another Sonderkommando unit then arrives on the scene, and Saul sneaks on to it. The truck heads to the nearby riverbank, and the prisoners are ordered to put the ashes from the crematorium into the river.