This is an interesting question to ponder on a rainy, post-holiday, Monday; although my reply here is kind of an "anti-answer" (or something like that) because I'm assuming the "writers" you have in mind are those who are creative and write fiction or poetry.
I have close to zero creativity as far as I can tell and write only non-fiction and the occasional verse/poem that, most of the time, I've only written because I was bored, went looking for a subject to "challenge myself" on, and tried to see what I could come up with as a poem. The exceptions are things that I've written with my children in mind.
In any case, I pretty much see nothing, but if I were to try to describe what I do "see" it would be a bunch of "mental files" that stay closed unless/until I'm trying to find something in one of them. When I write non-fiction I'm operating from one "current" file. There's also an "incubating" file that contains a bunch of other "incubating files" (and those are within the "current" file but not open at the time I'm writing directly from it, unless I go looking for something in one of them). All the stored, rather than active, files are somewhere at the back of my mind (against the "back wall of it", I imagine). All the files that are stored are stuff that has already been processed if there was ever a time when personal/emotional type was involved.
While I write what I "see" is "nothing but air" or transparent (the best way I can describe not seeing anything at all). If I imagine what I'm seeing if I look harder it's printed words streaming out of any one particular file. The ones from the current file would be more like typewritten words with more "body" to them than the words from SOME closed files have (depending on whether the closed file is from my "black-and-white" of stuff I read/learned that other people wrote or not).
The stuff from those old/closed files "looks like" words on barely visible ribbons, streaming "float-ily" into the "current" file for inclusion. It's all drearily and un-interestingly unimaginative. :/