I suspect the garter belt is some sort of (semi?) spandex satin. It doesn't look as stretchy as swimwear, but it's not the same satin as the corsets. I've seen another photo of Columbia in floorshow where hers looks stretched, too.
As for the straps, my best guess is that the red is pleated before being sewn onto (what is presumably) elastic. At least that's the method I'm going to experiment with first. I have a few more screen caps from the pool scene, and those definitely look pleated.
