as we know in the case of entire civilian populations like the Jews and to some extent the Roma (later on), selection for able-bodied workers took place at places like Auschwitz with those too old, young or sick for labor were gassed, or before deportation when killing was done by shooting or at dedicated extermination camps. The non-Jewish prisoners were a more select group. I can imagine people like captured resistance fighters would be around fighting age and could be sent to a labor camp. But political prisoners or other target groups could be older or infirm, etc. What was done with them? Were there regular concentration camps or sections of labor-concentration camps to keep prisoners such as this? Or were they sent to regular jails? Or simply shot?
And on that note, in the case of the "common criminal" category of inmate I could imagine those fit for labor being picked out of local jails, but what was the ultimate fate of common criminals already in jail or imprisoned by local police later, namely in the occupied countries of eastern europe and non-vichy France and perhaps others that weren't run by an allied regime and under direct German administration?
Late reply, but i can give some answers to the first question.
Generally, all of the arriving non-Jewish deportees were registered and admited into the camp as inmates, irrespective of age. The only exception was so called "Police prisoners", which were Poles who were alleged resistance fighters or had commited other supposed grave "crimes". These had already been sentenced by the Katowice Gestapo court to death and were transfered to the camp where their execution would take place and were thus not registered as inmates.
When you look at prisoner photos you can see people in their 60's or 70's, typically Polish farmers or intellectuals. So they were in the camp until most of them were murdered by the SS/Capos(prisoner functionaries) or died of exhaustion/disease/hunger. This was particulary so in the camps existance from 1940 until spring 1943, where conditions for non-Jewish(except for Germans) inmates were not that much better than the Jews. However from 1943 onwards, things changed. Because of the German need for workers to keep afloat the war effort, certain improvements were made. Non-Jews could now receive food parcels from their families and they were no longer subjected to selections to the gas chamber. Some were also assigned to lighter work if it was apparent that they were incapable of performing heavy labor. So the survival rates of these prisoners were extended, including for the old and weak who would have otherwise perished.