This article gets into a lot of your questions. Here's an excerpt:
Nazi Germany maintained a supply of slave labor, composed of prisoners and concentration camp inmates, which was greatly expanded after the beginning of World War II. In Poland alone, some five million people were used as slave labor throughout the war. Among the slave laborers in the occupied territories, hundreds of thousands were used by leading German corporations including Thyssen, Krupp, IG Farben, Bosch, Blaupunkt, Daimler-Benz, Demag, Henschel, Junkers, Messerschmitt, Siemens, and Volkswagen, as well as the Dutch corporation Philips. By 1944, slave labor made up one-quarter of Germany's entire civilian work force, and the majority of German factories had a contingent of prisoners.
Notably, they dispossessed the inmates of everything they had and gave their businesses to those friendly to the regime.