Comment by MrAlmostWrong

While I can't speak on how the designer of this particular piece created the shapes you see, in Sketch I would create a shape (the flower petal) and then do "rotate copies". Do that a couple of times with the different size shapes and give each layer a different opacity (ability to see through the shape). Making it a black background with high opacity and layering them on top of each other provides for the cool effects.

Then place a rectangle background with a gradient on it that transitions from purple to green like in the graphic.