After the war and after the territorial changes resulting from this war, there was a massive "exchange of population". Germans were forced to move from new territories of Poland and Czechoslovakia to Germany. Poles were forced to move from Soviet Union to Poland. Ukrainians were forced to move from Poland to Soviet Union. All Germans from the new Kaliningrad region of Soviet Union (=former east Prussia) were moved to Germany.
The numbers of people moved in each case were millions. At the time when this happened (the years immediately after WWII)
there was no East and West Germany (as separate states. There were Russian, British, French and US occupation zones, but movement between them was possible). People who were moved were not
resettled on some well defined territory. They were just scattered over Germany.
(Same thing happened to other resettled people). Political
organizations of those displaced Germans who demanded return existed in West Germany for long time,
but I am not sure whether they still exist. (In Soviet Union and Communist Poland and Czechoslovakia, no political organizations independent of the government could legally exist).
This kind of massive population exchange had a precedent in the 1920th when a similar thing happened between Greece and Turkey. I do not know any earlier example. Later the same thing happened with newly formed India and Pakistan,
and on a smaller scale in many other cases.