How does this generate a Sierpinski Triangle?
(Assuming there are sprites in 3 different locations called “1”, “2”, and “3”.)

 when green flag clicked
set pen size to (1)
forever
	set[Point v]to (pick random (1) to (3))
	point towards(Point)
	move((distance to (Point))/(2))steps
	pen down
	pen up

That looks like an implementation of the chaos game:



Essentially, all that's going on is it's going halfway towards its destination. Assuming it starts at a random point, it could go to any other point, then it picks another point and goes halfway there; it keeps going at this, and yet never lands into a space that should be clear.

Video demo

As for why it actually does the Sierpiński triangle, I'd assume there's a lot more in-depth information on the prior linked Wikipedia article.

If the sprite starts at a point of the Sierpinski triangle, then after it moves, it ends up at the same point of one of the the sub-triangles