Carla's construction steps are manual, she has to draw some arcs to get a couple of points and then connect them by a straight line with the help of the straightedge. On the other hand, Daryl can draw a line in the computer and see which angle it makes respect some reference, then when he draws the second line he just add 90° to the angle of the previous line, to make them perpendicular. He doesn't need any auxiliary arcs or points.
