Which wall structure is meaningfully vapor-permeable?

Our general contractor explained the system like this. The window vents go into the windows. An exhaust fan goes into the bathroom. It runs constantly, it doesn’t need much electricity, it just hums quietly. It removes air from the bathroom, creating a slight negative pressure, and fresh air flows in through the vents. Always, day after day, 24 hours round the clock. Brilliant, cheap, simple, maintenance-free, mold-free, everything-free. He installs the system as standard in his houses. Those who want more also get a [Kontrollierte-Wohnraumlüftung] from him, but he clearly says that it’s not necessary. For houses with an upper floor, of course you take two fans, one upstairs in the bathroom, one downstairs in the guest WC. K.

Brilliant, cheap, simple,

Cheap: yes
Simple: yes
Brilliant: ? In the past, people simply said "leaky windows" to describe it

No, there was a draft at a leaking window. There is no draft at a [Fensterfalzlüfterfenster].

Sounds really good, may I ask who the [GU] was?

We also have exhaust fans in the wet rooms and window frame ventilators on the windows (1x per room). On our windows, the seals on the left and right sides have been cut open (lower third about 10 cm long). At the top of the window frame, there is a rubber membrane. Ventilation-wise we have no problems, but clearly it is "more complex" than with controlled residential ventilation. There are some nuances you have to be careful about when ventilating. We also tried to use as few "technical aids" and as little building material as necessary. explained it well and pragmatically.

We also have exhaust fans in the wet rooms and window frame ventilators on the windows (once per room).
On our windows, the seals on the left and right are cut open (lower third about 10 cm long). At the top of the window frame, there is a rubber membrane.

From a ventilation point of view, we have no problems but clearly it is more "complex" than with controlled residential ventilation. There are some details you need to pay attention to when ventilating.

We also tried to use as few "technical aids" and building materials as necessary. explained it well and pragmatically.

What are those details, if I may ask?