Why do some airplanes have the wing tips folded up at about 80 degrees and some planes do not?
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Basically, the air under the wing wants to escape toward the low pressure air above the wing. The problem gets bigger when your wings are swept back as the air has more time to do it.
A simple solution is to make the wing longer in the initial design.
However, if the plane already exists and you want to make it better, you can’t simply strap one meter of extra wing. The wing is already designed and any extra load you put on the top will act as a lever and stress the wing too much. On some planes the added dimension is also a problem for parking.
You can retrofit a vertical obstacle to the escaping air. It doesn’t extend the wing width so it doesn’t it add “lever” stress. In theory, you want it to face downward, but this could touch the ground in certain difficult landings. So there is the option to make it go up. If it goes up you need a bigger addition to compensate the reduced effect.
Another solution is to make a small addition down, using all the clearance you have toward the ground, and add a second one on top to improve the effect.
Last, to reduce drag you better reduce sharp intersections to the minimum. So some are made with a wide radius curve at base, and then go straight later. (Surfaces meeting at 90degrees do make vortexes, that’s also why you see the wing root intersecting the fuselage with a smooth flare instead of going straight into the fuselage)
This cover more or less all the shapes you see.
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