There are a lot of muscles in the back with different function and mechanics. I will share with you detailed information about those muscles and their function to that targeted development you looking for.
The traps are divided into three parts. The superior or upper region originates under the skull and attaches to the scapula. This portion elevates the shoulders. Barbell or dumbbell shrugs are targeting that part.
The medial of middles section function is to pull the scapula toward the middle, squeezing your shoulder blades together. Exercises like lat pull down or rows targeting that part of the trapezius.
The lower section function is to pull your shoulders down. This muscle is engaged when performing a lat pull down.
It is important to understand this muscle function if you want to build thickness in your back.
Lattisimus dorsi is V shape broad and flat muscle. This muscle origin attachment point is the lowers spinal, the lower ribs and lumber and its insertion point of attachment are on the front side top of the humerus. The function of the muscle is to bring your arm towards the mid part of your body. Another function is to flex your shoulder bringing the humerus close to your side. Think of a reversed grip pull as an example.
The infraspinatus is located between your traps and rear deltoids. When training rear delt you train the infraspinatus as well. That is why training rear shoulder is important for the overall development of the back.
Terres major attaches at the back of the humerus like the lats but the other point of attachment at the scapula. The muscle has the exact same function as the lattisimus dorsi muscle. This is small but extremely strong muscle. Many trainees using this muscle when training back without realising it. This could restrict the lats overall development.
Spinal erectors function is to straighten and hyperextend your back. When performing a deadlift those muscle role is to work isometrically and keep the spine in its neutral position.
Having this information you can learn how to target specific muscle or muscle group and shaping your body the way you want it. This knowledge can help you to visualise the working muscle and build your muscle-mind connection which is fundamental for targeting muscle development.