How the crop is chosen
A crop keeps the largest area of the new shape that fits inside your video. If the shape is wider than the video, the full width is kept and the top and bottom are removed. If it is taller, the full height is kept and the sides are removed. A shape your video already has is not offered, because it would change nothing.
The kept area is not scaled up or down. A 1920×1080 video cropped to a square becomes 1080×1080.