You can read, in a recent news article, about how Jewish prisoners had to put the bodies of their fellow Jews into a furnace at Auschwitz: http://www.foxnews.com/world/2017/10/11/buried-letter-recounts-auschwitz-prisoners-job-burning-bodies-fellow-jews-in-death-camp.html
Marcel Nadjari, a Jewish prisoner in Auschwitz, wrote about his experience of having to take bodies from gas chambers to the camp’s crematorium (REUTERS)
I took a photo of this same oven when I visited the site of the Auschwitz main camp in 1998; my photo is shown below.
Note that the recent photo of these ovens shows that the wall behind the ovens has changed. It is now a brick wall.
The crematorium in the main Auschwitz camp, later designated as Krema I, was first put into operation in September 1940; prior to that time, bodies were taken to Gleiwitz to be burned in the city crematorium.
Initially, this crematorium contained two ovens which each had two openings, called muffles or retorts. Bodies were shoved inside by means of a device shown in the photo above. The ovens were deep enough to hold two bodies, placed end to end. A third oven was installed at the end of 1941. Krema I was in operation until July 1943.
The two photos above show one of the two ovens which were reconstructed by the Soviet Union in 1947 when the main camp was turned into the Auschwitz Museum. The small doors at the bottom were for removing ashes.
The blueprints for the crematorium at Auschwitz I show that there were three ovens when the crematorium was first put into operation. One of the ovens was later removed.