For me the serve is about: forearm, hand, chest and back muscles.
The other muscles like your shoulder, bicep, tricep etc. Are all stabalizer muscles so you don't throw your shoulder out or mess up something up.
I don't think loose is a good term. Natural is a better. Do you think about walking, or do you just walk? Do you force yourself to walk, maybe if you were crippled but as a person who is 100% healthy. You walk without giving it a thought, same with the serve. I serve without giving it thought, and it's natural. Thus my body is "loose."
You think about where you're going to walk to. But the process of walking, you don't give it a thought. It's just something you do. Same with the serve. You don't think about the motion, it should be natural and come naturally. And the thought part is just what you want to hit and where.