Birkenau was begun in 1941 because Auschwitz 1 simply could not cope with the influx of prisoners. It was begun for political prisoners but by 1942 Hitler had decided to exterminate the Jews so Birkenau was refocussed to the efforts of a labour camp and extermination camp.
4 crematoria were built as gas chambers and incinerators to kill as many people as was possible. The first one, Bunker 1, was called Red house. The second a little later Bunker 2, was called White house.
Crematorium 2 had been designed as a mortuary, but by installing gas tight doors and ventilation to remove the gas afterwards it was turned into a gas chamber. It already had ground level incinerators which were to become useful. Crematorium 3 was the same but 4 and 5 were built from the start as gassing chambers.
The Nazis chose Zyklon B as it was very fast acting and should have killed the prisoners in seconds. However that’s only in set circumstances and in some cases people took over a half hour to die.
There is a story about how several hundred gypsies -another ethnic group targeted by the Germans- had a rebellion and attacked the soldiers with makeshift weapons. 3 soldiers were killed but the rebellion was quashed and all involved later gassed.
Birkenau saw World War Two’s largest ever death march. The Germans fearing the advance of the allies set off from Birkenau in January 1945 headed for Loslau. The SS killed large numbers of prisoners by starvation before the marches, and shot many more dead both during and after for not being able to keep up with the pace.
Here’s some more photos of Birkenau