Sheathing isn’t needed under a metal roof, only some nailers.
A ridge board isn’t technically needed either. Done like that for decades.
I may be wrong, but those metal straps probably pull the rafter ties/ceiling joists up at the center and transfer more load to the side walls, much like a truss.
As far as the exterior being clad with 1/4" panels, there are some new houses around my area which are only “sheathed” with rigid foam and some ply at the corners.
That looks like rough sawn oak. I’ll bet it lasts a whole lot longer than most of the junk contractors throw up these days. Sure, it probably doesn’t meet codes, but I doubt the Amish care. They know it will last.
Cameron is right , sheathing is not required under metal, neither is a ridge board. I see this construction all the time in cottage country in cottages from the forties and fifties. Some cottages have no waterproofing material under the barn metal roofs and some have a layer of feltpaper spanning the spaced plank. Most of them have been up for close to seventy years with annual snow fall of better than 4 feet. They may not be to “code” and despite the often written report comment " structure is consistent with the building practices of the time" I find most of them have fewer defects that will need major repair than the builder stuff put up since the seventies.