According to Timatic, the database used by airlines, in order to transit Düsseldorf or Berlin-Tegel airport in Germany airside, i.e. without clearing immigration, the airport authorities must be notified in advance by the airline.
I understand this is due to these airports lacking transit corridors (looking at this map of Düsseldorf, non-Schengen lounges are in dark blue, separated from one another by Schengen lounges)
So how is the connection actually organised at these airports? What happens once you land?