It's not actually the muscles, just a different part of the brain. The connections associated with the movement you want get stronger over a lot of repetition. If I'm not mistaken it's mostly the back part of your brain called the cerebellum that controls this, and as the slang term suggests, you don't have to actively think about it. Very handy for things you need to do precisely, quickly, and often, like throwing in sports.

Or stupid stuff like flipping on a light switch in a dark room. Close your eyes and touch the wall then look and you’ll be amazed how close to the right height you are!

It's the brain, not muscles. If you were to assemble 100 items, you'd get better at the end of it. It's your brain figuring out how to do the repetitive motions better and faster. It's tougher with sports because putts aren't always the same, throws aren't always the same. It's the things that don't change where you get the memory.

Everyone knows the "five" senses, but in reality, humans have a lot more than five.

One of those senses is called "proprioception," the ability to know how your body is positioned (and moving) without other inputs (like sight). Oversimplified, proprioception is essentially the brain's communication with the nerve endings that connect to your muscles (e.g., motor neurons).

But your senses are not fully developed at birth. Your brain has to work out the signals it sends and receives and build those up into a structured system. "When I receive these signals, my hand is here. Those signals, my hand is there. These other signals..." etc... The process of building that communication between your brain and your motor neurons is building "muscle memory."

Something that makes it a bit clearer is to think of hitting a growth spurt. Your sense of position, the interaction between your brain and body, is unchanged, but your body is now taller (or stronger, or missing a limb, or supplemented with a mechanical prosthetic...) so your brain's trained response doesn't match what you actually want your body to do, and you have to re-learn and adapt your brain's connections to your new body structure.

And to do that, you have to repeat a motion again and again and process the results, refining your movement to match your intent. Eventually, after enough repetitions, your movement will be more consistent. Perhaps even habitual. And that's muscle memory.

'Learned Reflex' may be more understandable than muscle memory.

I like the Piano (or I guess any instrument) for this. I know exactly where the keys are for whatever song I'm paying, but it's all subconscious. If I look and actively try to see where the next key is, I can't play. But blind, my fingers just know, after years of doing the same motions.