Through these camps:
1,100,000 Jews - 1 million perished
150,000 Polish prisoners - half perished
23,000 Gypsies - 21,000 perished
15,000 Soviet POW - most perished
Auschwitz was a series of buildings with pleasant accommodations for the Nazi officers and their families. Birkenau was a camp originally built using stones from several demolished villages. When the stones ran out they built row after row of timber buildings to house the inmates - who were not expected to remain long and never expected to leave.
The crematoria were dynamited before the Nazis left and many inmates were shipped out or forced to walk as the Germans retreated in front of the Russian advance. All you need to remove a few million people is some pesticide, a few not large crematoria, and no objections.