The shape of your muscles—their length, structure, and where they attach—are all determined by your genes and can’t be trained. When bodybuilders (or infomercial gurus who want your money) say they can help you get the “shape” you want, what they’re really saying is that you can build muscles in proportion to one another so they create a particular visual effect.
For example, if you want your waist to look smaller, you can build up your shoulders and lats so your body is wider on top. If you want thicker arms, you can focus on the brachialis muscle that lies between the biceps and triceps (reverse curls).
But if you have genetically short bi’s with flat muscle bellies and you want mountainous peaks like Arnold in his prime…you’d better go back in time and find different parents.