As far as Auschwitz is concerned, it is necessary to note that none of the doors on display in the small crematorium in the main camp are original gas chamber doors due to the crematorium having been converted into an air-raid shelter already in 1944 (by that time the provisional gas chamber there had long ceased operation, the absolute majority of the mass killings took place in Auschwitz-Birkenau), and then was partially reconstructed after the liberation, so you can see a confusing mix of flimsy wooden non-gas-tight doors on the inside and air-raid shelter doors on the outside, none of which however tell us anything about the original state of things.