the trapezius muscle is one of the least understood muscles in regards to functionality. contrary to what most people think, doing shrug is not going to be doing that much good for you and you need to widen your exercises. in this video.
although I do not go through the variety of exercises, I explain the fundamental knowledge about traps anatomy and the way it functions. the traps muscle is made of mainly three fiber directions: upward(diagonal), medial, and downward(diagonal).
the upper fibers or upward fibers help in the upward rotation of the scapula or shoulder blade, which can be done by shoulder side raises. they also help in the elevation of the shoulder(shrugs), which can be performed by shrugs but it is better to angle the bench a little bit as mentioned in the video. also, the upper fibers assist in neck extension and lateral neck flexion as well, but it is not their main function.
middle fibers help in shoulder retraction(bringing the shoulder blades close to each other or pinching the scapula together). it can be done by exercises like back flies or dumbbell rows and even barbell rows, just make sure the scapula is moving along in the exercises and is not fixed.
at last, the lower fibers help in stability and keeping your shoulder blade depressed(opposite movement of elevation of shoulder or shrugs). this muscle gets activated during lots of back exercises but one of the best exercises for that is parallel Y-press.