Entry level SUPs, such as the ones sold in quantity on Amazon and at Costco, are made with a single thin layer of plastic coated fabric on the top and bottom surfaces of the board, and usually two overlapping strips of material to connect the top and bottom sheets of fabric and hold the air in. More advanced SUP constructions have additional layering of materials that will keep you better protected from air leaks, withstand more intensive use, and result in a more rigid board that will perform better.
There aren’t many resources that go into board material and construction details in any depth, so reading this explanation of inflatable paddle board construction methods would be a good start to understanding the different material layer layups use in inflatable paddle boards of different qualities and price points. Beyond the integrity of the pressurized core of the board, there are many places where cheaper brands save on materials. For example, deck pads may look comparable in product photos, but they are made with varying foam densities, which affects how much cushioning they provide and how long they last. Handles may seem like an afterthought, but a carefully designed handle with a removable cushioned grip can make a big difference in comfort when you are carrying your board to the water. Similar tradeoffs between cost and quality affect every component of the board and its included accessories.