The first time I saw something like classic guitar picks was in college. It was used by a friend who was very adept in playing the guitar. The acute isosceles triangle looked like just a piece of triangle material to me and I didn’t know that a guitar pick can be made of nylon, plastic, steel, glass, wood, agate, carbon fiber, and many more.
A guitar pick is gripped usually by a thumb and index. Other guitarists do it in another manner comfortable to them like using the thumb and middle finger, or using three fingers.
A guitar pick is used to effectively control tone and articulation. The technique in using it can already be learned online with ‘how to’ instructions.