>what the Hungarian Jews in Auschwitz did during their stay
The chemical industry had a lot of work in Auschwitz itself. It still does, the Soviets took away the most advanced instruments but the Poles put it to good use and today it's one of Europe's foremost suppliers of synthetic rubber. There's no mention of the Hungarian deportees working there in the book, though.
>I think Germany had the best interest to "recruit" all the usable workforce she could
That's what it eventually realized after a lot of initial wastage, though the success of economic integration varied: Himmler's workers couldn't be assimilated as well as Sauckel's. But at the same time with scarce food populations were a liability. So the synthesis which took years to achieve was to conserve the useful working population but not the rest.