Complex sentence: The small number of Jews who were not killed immediately became its "Sonderkommandos", labor units who were forced to bury the victims' bodies in mass graves; later they also were made to cremate corpses on massive open-air pyres.
Split into simpler sentences: The small number of Jews who were not killed immediately became its "Sonderkommandos". These labor units were forced to bury the victims' bodies in mass graves, and later also cremate corpses on massive open-air pyres.