Question on how pros serve?

Muscle has little to do with a fast serve. If you have good racquet head speed, then you will serve 10 times better than some super muscular person who tries to use just his arm muscles to generate power and speed.

most of a tennis player's general power comes from the legs and the core. if simon has a good load on his serve and on his forehand, he can generate more power than someone to uses weights all the time get stronger arms.

its all in the racquet head speed and you get that by using your legs to jump into the ball and your upperbody rotates through the ball letting the racquet release and crack like a whip when it hits the ball therefore giving it alot more power than it would seem possible. the rotation is key on the forehand as well.

Most of the power of a serve comes from fast racquet head speed.

you have to have a cooperating kinetic chain. it starts in your toes and then calfs, knees, thighs, waist, core, sholders, tricep/bicep, elbow, forearm, wrist fingers. you have to use them all to feel the energy come up your body to hit up as hard as you can on the tennis ball. rackethead speed is key, but its not the arms that do all of teh work. its the legs jumping and hips rotating. you doing either of those alone wont help. you have to work on timing every bodypart to get your full power

Its more than just raw power. Its mostly physics and kinetic chains and stuff. All the power comes from the body basically being a human spring.

Racquet head speed determines the ball speed.

Legs and hip. A good rotation and technique can be a lot more powerful than big buff guy with bad technique and rotation.