There are several ways to do this. One is to animate the shape layer or mask path. The other is to create layered artwork in Illustrator with arrow shapes that are longer than needed, then must animate. If the artwork is already created it would help to know how it was done. If you want to create the shapes from scratch then you can simply copy each of the arrow shapes using the pen tool in AE, or by using other drawing tools. Another solution would be to create a triangle and a rectangle, then animate the position of the triangle and the scale of the rectangle. The only thing that is kind of constant in the project is that each layer that is a different color should probably be unique.
If you have no idea what I'm talking about the check out the help files on animating shape layers and animating paths.