Paper airplanes are generally symmetrical, which means you can begin by folding the paper in half and using that fold to determine where the nose is. Unless you mean something else?
I'm talking about the leading edge of the wings, where they meet the fuselage. How far up that is from the tip of the nose is often not based on any existing edge or fold.
The fold that separates the "fuselage" from the wing? Not the leading edge (except in one point), but you are right, that is the one where many otherwise non-arbitrary planes make an exception. My favorite designs, front heavy with a decoratively complex leading edge would usually have plenty of potential guidepoints for the fuselage fold though. Good times, bringing up those memories makes me wonder when exactly I stopped.
Yes, that fold, but specifically that leading point of it. Often the back can have a reference point, like having the edge of the wing meet the bottom rear corner of the fuselage. But yeah, the fold in general is often more arbitrary. But not really; you would try to fold a plane as consistently as possible, so something like 'begin the fold 1/8" from the tip of the nose' isn't arbitrary at all.
Similarly, you often don't want to fold right to a crease, as it can cause bunching. It's often a better idea to leave a mm or so of extra space for inward folds. Doesn't make the design less pure or anything though.
