How does one know?

I will give you a hint: what you can see in concentration camps as a gas chamber is just a reconstruction. Most of the evidence is “he said, she said” AFTER the war and usually carefully curated by Soviet / Jewish propaganda. Auschwitz does not let independent researches to take samples around the camp.

this interview says it all. I’m too lazy to translate it to English, but if you are very curious you can download and AI translate it.

How does anyone saying anything tell you what really happen? Surely there is another person who would argue a different perspective.