Too many ways.
Those who were too young, old, or sick to work in the concentration camps were separated from the rest and almost immediately shot. They sometimes had the Jews dig a giant hole, only to line them up at the edge of it and gun them down; they would fall into the grave they just dug for themselves. They were gassed in droves in the "showers." They were burned alive in crematoria. They were sent on death marches without proper shoes, or sometimes without shoes at all, and sometimes in the winter. Some were used for medical experiments. They were beaten, starved, and hung.