What was the Nazi's plans for the US if they managed to (impossibly) conquer the US? Would they divide it into several different countries to make occupation easier? Would they set up a puppet state? What were their plans?
They probably never planned on that, recognising how impossible it would be. But if we speculate, we can always look to occupied Europe (Western Europe in particular; the East woudn't do well for a comparison).
Where feasible, they placed the local fascists in charge (relatively speaking; of course, they still held the power). This was the case in Flanders/Wallonia (Belgium), for example. Yet the US, like most countries, lacked a strong fascist movement. There they'd probably settle for the second approach.
In these cases, the Germans desired "respectable" government figures that could get some recognition both abroad and from their own people. This typically meant leaving the locals in power, as long as they did what they were told and were suitably to the right (Vichy France, Denmark).
In the US, Roosevelt or Wallace would be out of the question. Perhaps they'd "abdicate" in favour of someone more suitable; the Axis might press some suitably conservative figure, such as Joseph Kennedy, there. Though it's hard to guess, and I'm no expert on US politics...
There'd likely be extermination of Jews if the invasion was after '41. The other minorities I'm less sure of; the Nazis likely wouldn't think Puertoricans subhuman. Blacks is also a grey zone; although they were
Untermenschen in the Party line, the Nazis frequently ridiculed the US for their Negro lynchings as part of their propaganda. Perhaps they might be kept as workers (=slaves).