Hi thanks for asking, what looks like panelling is actually a much thicker wood material called "car decking" it is almost 3" thick and double tongue and grooved which is why it can span that distance. It is forms the floor of the loft area above it. This particular product is laminated in offset layers to create the tongue and groove. Old tongue and groove car decking was made from single lengths of milled lumber. This laminated product is that much stronger again because of the adhesives.

Where can I purchase the car decking? I have been looking for it forever. Thank you. Beautiful.

Just for the sake of offering suggestions...
We've milled prefinished "flooring" in a variety of species and colors over the years, in a thinner "profile" making it lighter and easier to attach to a ceiling (or wall) which has been "sheeted" with a plywood or OSB layer. Another approach is the use of "stringers" attached to the ceiling, which allows "blind nailing" of flooring material or something else in a T&G.
Really great for adding another dimension to an area or room!