It is
not only a subway station.
I have found
this plan of the station, the acces to the subway is probably in the sub level ("Untergeschoss") - "Erdgeschoss" is "ground floor" and "Bahnsteigebene" is "platform level". "Schließfächer" means "baggage locker", see in the legend.
This graphic ("Alex: ÖPNV in fünf Etagen") is an overview of the stations levels.
A google picture search for "bahnhof alexanderplatz innen -hbf" gives pictures of the interior.
Edit: And of course, in our days, people put
films of the
station online.
Man, TL 8 is really amazing.