I can’t even imagine how this could be done. Nazi concentration camps would have had trouble killing that many in 2 days.

that's because they weren't shooting crowds already assembled in the streets and going into hospitals nationwide to find the injured. Nazi Germany was aiming to maintain plausible deniability in the concentration camps for as long as possible, while parallel competing plans for what to do with the population were being explored and failing. (there were other solutions before and alonside the final solution)