In total, the Nazis established 356 ghettos in Poland, the Soviet Union, the Baltic States, Czechoslovakia, Romania, and Hungary between 1939 and 1945.
They were the first step before they started sending Jewish people off to concentration camps and they also removed them from the outside world, which is what the Nazis wanted.
In most of the big cities in Germany itself the Nazis herded the Jews into designated apartment blocks and houses after 1939, but these were not walled in like the ghettos in Poland.
Nazis were not the targets of the holocaust. I suppose it might technically be possible to answer the question anyway, but you run into the difficulty of distinguishing actual "Nazis" from people who...