The Germans built huge gas chambers that looked like factories. They then told the prisoners to get ready for a shower, but little did they know that in a few minutes that they would be gassed. These mass rooms that can hold up to 700 people. The most well known concentration/execution camp that used the gassing technique was Auschwitz.
Starvation
The Jews hardly ate, it was usually around a tiny piece of bread and little amount of soup. With that, the Jews would work all day and after their day of work, they would get beat and got weaker and weaker, which didn't help the fact that they were starving and more prisoners were dying each day.
Torture
Not only did the Jews get gassed and starved, but they also got beat, a lot. After a good month, you were so weak and skinny that you were only flesh and bones, no fat or muscles. Since weak Jews were easy to hurt and break limbs, the German soldiers would come out and beat them or throw stuff at them or if they did something wrong just shoot or hang them.
Guns
The Jews were not free from guns. If one Jew was to disobey one of the soldiers, or not follow a rule, they would stand at the edge of a pit and get shot right into the pit. There were also soldiers in towers at each corner and if you were caught trying to escape you would be shot immediately.