Answer:
They gassed the old people, women, and children 15 and below. After they were gassed with poison they were just carlessley thrown into the creamatory.
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Cremation happened in two methods; the main one was in the crematoria, the ovens were no bigger than crematoria where bodies are cremated today, but as they did not use coffins, they could fit up to three (depending on size) bodies at a time. During peak periods they had to use outdoor cremation pits to cope with the volume.