The word “emphasize” is key in your question, as it’s important to note that no muscle works in isolation (that is, no muscle works alone separately from others and no muscle head works alone separately from the rest of its constituent muscle) under normal conditions. The reason certain exercises can emphasize specific areas of a muscle more than others is simply a matter of movement mechanics, such that muscle attachment points, various angles of pull, body orientation, and joint involvement target certain muscle fibers more than others.
For example, the long head of the triceps originates on the shoulder blade, whereas the other two heads originate on the humerus, so movements that involve extension of the shoulder (e.g., triceps kickback), rather than pressing movements (e.g., overhead triceps extension or bench press) hit this head more. Another example is the quadriceps in which three of the four muscles originate on the femur and insert below the knee. However, the fourth muscle, rectus femoris, originates on the hip bone and not the femur, so it is not as involved in knee extension when the hip is flexed (e.g., when performing a seated leg extension on a machine) like the other three quadriceps muscles.
To target all parts of the each muscle consistently, perform exercises for each major muscle group in various planes of motion and with the body in different positions (e.g., flat bench press, incline bench press, and decline bench press). However, it’s important to realize that a muscle (as a whole) and separate muscle heads cannot be isolated during an exercise movement. Adding multiple exercises for one muscle group increases the volume and intensity of the workout and is considered an advanced form of resistance training. In other words, beginners and those who are short on time can get away with performing only one exercise per major muscle group and still reap the benefits of regular resistance training.