While movable flaps are certainly a factor in providing a radar return, smooth wings aren't the ideal from a radar perspective - hense the distinctive Tesla CyberTruck^TM appearance of the [F117](https://en.wikipedia.org/wiki/Stealth_aircraft#/media/File:F-117_Nighthawk_Front.jpg].

If you have a flat surface, it'll only return radar waves in one direction (which is unlikely to be where the receiver is) whereas a curved surface will scatter it in lots of directions, meaning some is likely to end up going back where it came from to be detected.

The worst case scenario is something that forms a retro-reflector, such as a corner reflector or the radar equivalent of a cat's eye so they're careful to avoid those.

As a thought experiment (or a real one if you feel like it) have a friend hold up a Christmas tree bauble and a similarly sized compact mirror in a dark field. Shine a torch at them, and see which you can see more easily.

The ball will have a sharp spot of reflected light on it, and the mirror will (almost certainly) not reflect back toward you and be seen.

Of course, if you happen to angle the mirror just right, then it'll reflect a whole bunch of light back at you, but of course they don't use mirrors, they use the radar equivalent of VantaBlack so even the reflection you do get is minimised, but the difference between the flat and curved surfaces remains.