What happened at German killing centers during the Holocaust?

Many things happened. Jews, Homosexuals, Gypsies, African Americans,Polish, Russian and disabled people(mentally and physically) were gased and experimented on. They were gased in rooms that looked as if they were shower rooms.Nazis did whatever they pleased with their victims. sometimes they would cut people open while they were still living to see how their insides worked. Nazis especially loved experimenting on twins whether they were identical or not. If the citizens had no reason for the Nazis to take the twins, then the Nazis found a reason to be able to take them.