I am sorry I don't have a more thorough answer for you, but I know they picked the jews with labor skills (metal workers, craftsman, engineers, machine workers) and they either sent them to labor camps or they sold them to business (basically as slaves) to work for them. And as someone else said, the healthy were sent to work camps and the sick or old or disabled were sent to death camps. They did frequent checks at the labor camps to weed out the sick ones. The jews would even prick their fingers and rub the blood on their cheeks so they looked healthier and not so pale.
I recommend that you watch Schindler's List, an amazing true story of Oscar Schindler, a businessman in germany during WW2, and is historically accurate for the most part. It is quite graphic and dark, but you are writing about a very graphic and dark time. Liam Neeson and Ralph Fiennes do an amazing job in the movie. Hope I helped.
