Timber beam floors with perpendicular timber floorboards as indicated in the drawing below are common throughout northern Europe. The floorboards are nailed onto the beams and are quite substantial with a thickness of around 25-30 mm. The floorboards are typically connected with one another with longitudinal tongue and groove joints.
I wonder if the floorboards serve a structural function? Specifically, I could imagine that they could serve to distribute loads or to brace the beam structure.
As pointed out by Solar Mike the floorboards are indeed typically connected with one another with longitudinal tongue and groove joints.