re: How did the Germans know who was Jewish?

Jews in Europe typically lived in certain districts. Despite the cheap jokes, many ( obviously not all) Jews did actually look just like their Polish, German neighbors . So a system was put into place (obviously the star) that compiled their names and addresses as well as documentation they were required to carry. False papers were an instant death penalty. Those who had non- Jewish features, with the help of others, were sometimes able to hide and migrate to a safe border via safehouses . But it was pretty easy to round up a block of a neighborhood of Jews in traditional clothing.