TXL has a very traditional layout which has almost died out elsewhere: airside in the old building (A) is virtually non existent. Landside is the entire central inner circle, the outer circle, with the airbridges, is airside. Security is shared between a few airbridges, but there is very little space there after security, and arriving passengers are brought to landside very quickly. Each arrival from outside Schengen has its own mini passport immigration control. D terminal is a newer thin terminal, with security shared between the small number of gates there. C is newer still with two block of security. If you had a purely C arrival-departure there is a small scope for an airside transfer, otherwise basically it doesn't happen at TXL. This is not an airport with a large airside area with lots of shops and gates, it is pretty much the other way around.

Despite that, TXL doesn't cover that much ground, to get from A to D is only going to take about 5 to 10 minutes. A, B (bus section of A), C, D, E (which is below D) are all connected together by corridors, just A to C is a bit of a hike, more thanks to poor lane discipline.