i think it can be either way or a combination of being planned/unplanned. a painting can come very spontaneously, while you're working on it, or you can really plan out the entire painting from start to finish.
for me, personally, i have some sort of idea what i'm getting into about 40% of the time. the rest of my art is completely spontaneous and on a whim, usually figured out as i work on a project. or, while i'm working on one project, i may get an idea for an entirely different project.
here's an example of a mixed media piece that i had to plan out somewhat meticulously: https://www.etsy.com/listing/639812741/i-was-in-bed-for-a-while-but-9-x-12in?ref=shop_home_active_34
here's something that was very spontaneous: https://www.etsy.com/listing/584378205/a-l-l-o-f-c-r-e-a-t-i-o-n-18-x-24-in?ref=shop_home_active_15
what would you say from your own experience?