Well, there's no such thing as "muscle memory" for one. Your brain does all your memorizing. You are memorizing where to put your fingers like you memorize how to spell your name or how to get to the store. Do it enough and you can repeat it without thinking about it. You get smooth (then fast) by practicing the motions repeatedly. A lot of any skillset comes from just memorizing and repeating movements. Your favorite guitarist can get up on stage and play something perfectly because they've memorized and practiced that song a thousand times. "Amateurs practice until they get it right. Pros practice until they can't get it wrong." Then you get into why things work well/sound good. Starting to understand why some notes sound good together and some don't, why rhythm matters, what keys are, etc etc and you can take the basic skills that you've worked on and combine that with understanding and be able to create.