My house was renovated in '75 and most rooms have these ceilings. They seem to be made of gypsum and there are no visible cross seams. There must be some, of course, but none visible anywhere, so it might be tongue-and-groove. The longitudinal chamfer is rounded and not beveled. They are 60 cm wide. Does anyone know if this is available to buy new today? I haven't found any so far.
When was the house built?
Module house? We have the same ceiling that goes from wall to wall as the ceiling on both the upper and lower floors. After this, the internal walls are built so that the studs are under the drywall. Lift a shadow moulding and check if you have the same solution.