Unit 731 had vivisections (dissecting a person alive) in which scientists conducted experiments by testing people's pain tolerance via torture or inject them with diseases or toxic chemicals using prisoners (civilian or allied forces who are captured, but the majority of prisoners are Chinese). Japan today knows it happened but still remain in denial about the topic.
I don’t know about specific programs like Unit 731. But in general: People wrote letters. People came home and talked to their spouses. Concerning the Germans: A lot of people were involved. The Einsatzgruppen were staffed by ordinary police. The Waffen SS fought alongside the ordinary Wehrmacht which had their ugly share of atrocities. All these people wrote and came home and talked. And the concentration camps on German home soil were not isolated. People saw what happened there. People worked there as civilians. People who say the general public didn’t know are lying.
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