Upholstery issue with Bayliner

I am re-upholstering my Bayliner and to take off my side panels I had to tear into the upholstery to get at the screws. Every screw was under the upholstery and under the foam (wood screws). I was wondering how they installed these panels with the screws being where they are. The panel upholstery was stapled from behind, as normal, and the screws where not near creases of any kind. So how did they do it? I will be installing the wood bolts that dig into the wood from on side, same as my rear seat back rests.