This means that there <em>was</em> selection among the children and elderly (whether or not they were fit to work, whether they were sent to Auschwitz or Birkenau) but among those of normal working age, there was no selection, and so they were sent directly to Birkenau to work.
"Other than" in this sentence means the same thing as it usually does (it is the same as "except for") but I agree, this is a fairly complex example.