Much of the footage that you see was done by the Nazis themselves. Their plan was to destroy the Jewish people and wanted to have evidence of how they had done so in a very thorough and efficient way. There was also a plan for Führermuseum - a museum whose purpose was to display a selection of the art bought, confiscated or stolen by the Nazis from throughout Europe during World War II. While ultimately the war ended before they could wipe out the Jews and build a museum, they had created a very significant amount of visual evidence of what they had been doing.