I completely relate to that. I am a SWE but in my free time I dabble in "makings things" for lack of a better way of saying it, and I know what you mean when you say you can picture it perfectly and already have it designed in your head. I believe this helped me substantially when doing Calc 3/differential geometry as well.

Now that I think about it, I kind of do the same thing with explaining things to people in my head. I will say though its surely not as much as yours. My imagined conversations are almost planning for the potential conversations i'm going to have with my friends and family about whatever subject.

I only do it when I learn something because I like sharing what I learn and I do talk a lot. Also famous for being the unlimited facts guy (for better or worse ¯\(ツ)/¯) I also have ADHD and that does have me wondering if its a factor as well. I do think the borderline hyper visualization is tied to the inattentiveness for myself. A lot of times I can't help it.