Drew, you are right, there are some differences both inside and outside.
Universal Mounting System, link - http://www.autohome-official.com/en/products/roof-top-tents-characteristics/mounting
Big advance when it came out a zillion years ago, before that you needed a expedition basket to support your RTT.
Outside differences - While newer tents have adopted the mounting system's 'C" channels, they just bolt these channels to the base, an easy solution but not as strong. In a Maggiolina, the "C" channels are moulded flush into the base resulting in a completely flat surface. The importance of this is that the bars and base are integral as the base prevents the bars from bending and vice versa. When you tighten the "U" bolts, it becomes a unit. You might be able to replicate the system on other tents by adding filler pieces between the base and bars, supporting the bars and making a stronger system. On the older AutoHome tents you wanted to have the bars at least as wide as the tent to support the outer edges. While the newer tents don't need this, it is still a good idea.
Inside differences - because the bars are an integral part of the system the base can be built lighter. Mike is correct that the base is a composite similar to a Boston Whaler but really it is more like an America's Cup catamaran where the fiberglass is bonded to an airex type foam core. Fiberglass is not very rigid if formed into a flat panel, but strong if made into compound curves or if made into a composite with a bonded core separating the glass panels. If you use carbon fiber bonded to a aired foam core, the rigidity is off the charts, and this is what they do for the Americas Cup boats.
So, bottom line, all you need are two crossbars. We would add a middle bar for heavy off road use. The middle bar would be set up slightly higher so when you crank down on the bolts the base would flex like a spring. Then when your vehicle flexes the tent flexes too so it is silent up there as it is stressed. This was for the vehicle, not the tent.