Treblinka was an extermination camp, not an ordinary concentration camp. The idea was to kill new arrivals as soon as possible. It served no other purpose. A very small number of new arrivals were made to help with the extermination process itself. In particular they had to dig mass graves and sort the belongings that the newly arrived and gassed victims had brought with them.
Until the winter of 1944, Bergen-Belsen was a fairly decent camp. The inmates, purposely not called prisoners, were used as bargaining chips for the Allies. In February 1945 typhus broke out and many people were transferred to Belsen as the Allies drew near.
The most common method of transport by far was by train.
The camp was run by the SS, who did most of the killing. In the early months of 1945 there was also a severe typhus epidemic in the camp which killed many of the prisoners.
Auschwitz Birkenau, it acts as an death camp but it's an extermination camp.