Exercise. The hand contains many small muscles between the bones that contribute to hand strength and dexterity. ANd like any muscle they will get stronger and bigger with exercise. However, these muscles are delicate and can be strained or injured with overuse. If you really need to improve hand strength I would suggest a referral to a hand therapist who could advise you on the best way to improve grip.
Muscles of the hand. are small muscles that do not have a large cross sectional area and it is hard to "pump u up" there. One can gain strength and endurance but size is tough. The flipped is if the muscles are atrophied one needs to find out why they are. Still squeeezing a tennis ball to special hand grippers do work but it takes perseverance Hand is a good indicator of overall upper body strength /that works too.