First impression Division or question regarding walls of the upper floor

Regarding wet rooms: Is that more about the advantage concerning the installation of pipes?

I didn’t pay attention to the layout at all.
Advantage is a bit too "positive" of a term.
The wastewater has to go somewhere. Depending on the structure, you can somewhat route some of it in the floor. Toilet very limited. But somewhere it has to go down. If there is nothing there...
Your parents' bathroom would be very difficult. And even if you take the outer walls, you end up with the wastewater pipe through the dining room...