I just replaced a couple zippers in my girlfriend's (commercially made, and old) pack. It was blindingly obvious the order in which the seams were sewn, at least for the seams that are dependent on each other. There were a couple of surprises (a zipper that seemed to have been sewn to the pieces of the main pocket with no sliders, the two pieces sewn together where the zipper wasn't going to be, the sliders installed along with stops, for instance. I wouldn't copy that: it made replacing the zip very complicated, and I'd have had a very hard time getting the sliders on that late, where access is limited. If they had some machine that made inserting sliders easier (or just someone who did more in a day than I'll do in my life), it probably saved them three or four handling operations, which is real money.) Where the seams are dependent on each other, it's either not important, or it's pretty clear to someone with some experience that doing Y before X will make X harder, so do X than Y.
There are many things that are not possible to figure out from just inspection of existing equipment. You can't, for instance, work out why they're using a particular fabric, and not a different one that you'd think is equally suitable. The answer could be all sorts of things: they didn't know the other fabric existed, they couldn't source it (cheaply enough, or reliably enough, or at all, or in the right colors, or ...), it was too hard to sew properly, or it could be that they discovered in field trials that it isn't really suitable. that has to come from experience, and that's what makes fora like this so valuable.