What is the structural/chemical difference leading to different shapes in facial/head/pubic hair?
hair shape texture is really easy. the biggest effect is the shape, size, and angle of the hair follicle. Hair with different cross sections have different properties. If hair is stiffer in one plane than other it will tend to bend along the weaker plane. Larger follicles will of course produce thicker, thus stiffer hair and the angle of the follicle can influence the hairs cross sections as mush as the follicles cross sectional shape can. Note the chart below is simplified triangular and kidney shaped cross sections also exist.