It is almost impossible to comprehend killing of thousands of people.
Try better to relate to a killing of ONE INDIVIDUAL. In this case, One named Dr. Kurt Wieluner.
Dr. Wieluner was deported to Auschwitz on 16 October 1944. His transport of 1,500 Jews arrived there the next day.
Dr. Wieluner was murdered in a gas chamber later on. ( Source Auschwitz-Memorial)