As far as I remember, the tattooed people (numbers, no barcodes ffs) were in the Stammwerk I in Auschwitz, and there were no operating Gas chambers, they „rebuilt“ one after the war - there were no gassings. It was a concentration camp, and that was horrible enough.
I have my issues believing this whole Zyklon B story and massive scale of gassings in Auschwitz II birkenau. I visited to a lot of KZ‘s, including Ausschwitz.