Nazi philosophy grew out of 19th century ethnology which was heavily influenced by Darwinism. They believed the proto-indo-european ("Aryan") homeland was in northern Europe and that the spread of "Aryan" language was do to "Aryan" immigration to those regions and that in ancient times those regions were led by an "Aryan" "master race" who ruled over the "lesser races" do to their natural superior genes developed through generations of natural selection in the harsh northern climate.
To be clear, I believe that is all BS but I wrote all that to clarify that the Nazis didn't steal the term Aryan from the Hindu, it's a term used by the Indo-Iranians to self-designate which was erroneously misattributed to the proto-indo-europeans by early European ethnologists.