On arrival in the death camps the Jews went through a
selection process: those who could work and those who could not work. The latter group was immediately killed in the gas chambers.
How?
Victims were told to undress for a disinfection shower. Once in the shower, which had actual shower heads, gas was poured into the room, killing everybody inside.
The gas was Zyklon B, a pesticide. These are crystals that, when mixed with oxygen, spread a poisonous gas.
Afterwards, the bodies were cremated in ovens.