The Nazis confiscated the money and possession of the people it conquered, and it also confiscated the possessions of its own Jewish citizens : this contributed greatly to their wealth.
Before the war they motivated the whole country to work for the country: so people worked a lot rather than focused on their private life - this means that the unemployment was low but also the cost of work was not too high. They also invested a lot into the infrastructure with borrowed money (budget deficit).