You have to see Birkenau (Brzezinka) with its train unloading ramp, and the wooden stables to fully understand the horror of Auschwitz. Some of the wooden barracks once served as stables for just over fifty hourses - then after some modification they had to accommodate up to 1000 prisoners. The maximum number of people who lived in any one time in Birkenau concentration camp was up to 100,000 people (1944).
If you visited only the main camp (Auschwitz I) you saw nothing. The main Auchwitz camp is just a museum located in the descent looking brick houses (see below), who were once a part of the concentration camp and earlier (before the war) they served Polish military. Visiting the museum gives only some ideas about the horror, with stocks of shoes not only adult shoes but the small ones from children, tones of suitcases, hundreds of walking sticks and artificial limbs (probably still after WW I veterans) or the haircloths made out of human hair. Auschwitz was a real "factory" of death which used anything possible from humans - even the fat to make a soap.
For many years after the war the number of people who were killed in Auschwitz was estimated at 4 millions. Then this number was adjusted to 1.1-1.5 millions. Why so much confusion? It is because people who were sent to gas chambers directly from the train ramp were never registered; of course this was the vast majority of people who perished there. So, although the number of transports could be accounted for - the amount of people in each transport could be only estimated. Four million as a total death count was first estimated by taking into account the "efficiency" of the gas chambers. But its narrow neck that means the "efficiency" of the ovens where the bodies were burned limited the amount of people who could be killed each time. Fortunately, there were some days and even the weeks when no any new transports were arriving. The current number of victims was recalculated after taking into account the total number of Jews perished during the WW II everywhere in the world.
The majority of people who died in Auschwitz were Jews, at least 2/3 of them were never registered since at least about 75% of each transport were sent to the gas chambers (mainly elderly, children and women). The rest were sent to work as physically fit, among them also some women.
The survival rate in the camp was usually from 3 weeks up to 3 months. The total amount of registered people in the main camp and the subcamps include 200,000 Jews, more than 140,000 Poles (at least 80 thousands of them died there, Poles were the first victims of Auschwitz, before the massive transportations of Jews from abroad were initiated), approximately 21,000 Gipsies and 12,000 Soviet prisoners of war.
The majority of Jews who died in Auschwitz were not Polish Jews but the Hungarian Jews (over 410,000), Polish Jews were the second (above 200,000) and French Jews - at least 60 thousands. Polish Jews were "served" by several extermination camps like Treblinka, Sobibor and Belzec since Poland was the country with the largest Jewish population in the world.
There is a strange misconception that the only people who were sent to concentration camps except Jews were... gays and communists, maybe this was a true in Germany but not in the other occupied countries. According to the official records ONLY 48 prisoners in Auschwitz were imprisoned there because there were gays - from over 202 thousands preserved individual records (!) All of the gays came from Germany, 12 of them died in the camp, the rest survived.
The Poles of non-Jewish origin who were sent to Auschwitz and other camps were mainly political prisoners, spiritual leaders, priests, members of intelligentsia or cultural and scientific figures. Nazis wanted to get rid of the most affluent class of people who could influence the rest of Poles since Poles were considered the undermenschen good only for serving the super-race.
Soviet prisoners of war (POW's) were the worst treated people in the Auschwitz camp from all people allowed to stay alive. Their food ratios were the lowest, they died massively of starvation, there was even no grass left in their camp. The survival rate there was almost... zero. Soviet POW's were serving also as experimental guinea pigs, they were sent first to the gas chambers to test whether the chambers work properly. Soviets should "thank" for their unusually harsh treatment not only to Nazi politics but to the Stalin. Stalin and his regime treated all Soviet soldiers captured by enemy as the traitors of a Soviet state. If such soldier escaped and dare to come back - he was killed by Stalin people. So, no any international treaties about prisoners of war were ever applied to them by Nazi.
Gipsies were kept in one family camp in Birkenau. The conditions there were so terrible and unsanitary that Heinrich Himmler after his visit ordered to liquidate the camp completely and send all of them to the gas chambers. The same fate awaited Czechoslovakian Jews from Theresienstadt who were also kept in the family camp.
People in Auschwitz were executed to death not only in the gas chambers but also during many ecents of massive executions, for instance at so called death wall. Many of them died from hunger, starvation, disease, by torture or in special torture cells, by pseudo-medical experiments, as a punishment for escape of somebody else etc etc etc. But the official records almost never mention any violent deaths - as like the majority of them died from natural causes. Polish priest, father Maximilian Kolbe, now a saint, has given his life for another prisoner and died due to starvation in a special cell designed to it - as a punishment for the escape of other inmates.
This Way for the Gas, Ladies and Gentlemen (Penguin Twentieth-Century Classics, by Tadeusz Borowski, Barbara Vedder