Having said that, for me it's impossible to describe. There is a part of my brain that is putting things together (graphically, sort of) but simultaneously thinking dependencies, potential problems, optimisation, etc.. Then there's a part that is doing things based on experience and repetition - to be fair the second and third parts are more based on experience than anything else.
Aphantasia is the inability to visualize mental images, that is, not being able to picture something in one's mind.
There was a thread here on hn a while ago about this phenomenon and it seems even if one is not able to visualize things they can still be perfectly able programmers and even illustrators.