There is a maximum vehicle length though, and a maximum weight, the former ensures vehicle maneuverability, the latter serves road stability, bridge specifications, optimized no-waste parking provisions for pass-through parking spots in said truck stops edit: and ferry capacity. Ireland, Finland are connected by Ferry.
Manufacturing is pretty much internationalized, reusing parts and designs globally. Freightliner and Daimler are one company, for example.
Fuel economy is indeed a point, albeit the combo of trailer and truck define the overall fuel economy. As the trailer usually is equivalent to a standard 40ft container, the ideal truck has a boring standard form, too.
The text is German, the pictures are self explanatory-ish though wrt optimal and sub optimal wind résistance: https://en.vda.de/dam/vda/publications/FAT-SCHRIFTENREIHE%20...