I feel like I'm not comprehending the text if I'm not visualizing it to some degree. It's like I require the visualization to remember things. Symbolic/diagrammatic is also visual for me. Basically all of my sensory modes have to have a visual tag for me to remember them easily. I struggle to remember music without a lot of repetition. It induces the visual tags as I hear it, but it doesn't seem to round-trip properly. Maybe if I knew musical notation well enough to transcribe what I heard, I could do it that way and remember, but I don't.

Visualizing mostly happens naturally for me in the process of thinking. But more difficult material requires a slower, more conscious effort. I have to pause and think. That doesn't work while skimming, and it takes brain power away from other things. I feel like I'm not as aware of my visual field when concentrating, even if my eyes are open. I always had trouble keeping up with the lectures in school. I ended up zoning out and just reading the textbook. But I often watch YouTube lectures at 2x. This works because I can pause it if I need to think, or rewind if I missed something important while thinking.

When doing algebra, or refactoring code, I'm doing symbolic manipulation in my head in a visual way.

I think my imagination (and memory) might have been somewhat more vivid as a child. Now I can get away with lower-resolution abbreviations. But I also feel like I have better control now. As a child, I used to occasionally have chaotic visual thoughts that would sometimes become obsessive and hard to stop, like a day-mare. I can arrest such thoughts within seconds of choosing to do so now, and they're rare. I can also visualize three dimensions pretty well now. That took some practice, but it's a skill I started to develop while I was elementary-school age.

My visual imagination is pretty much constantly on when I read chemistry papers.  There's a stereotype that you read a synthesis or catalysis paper by (1) carefully looking at the figures, (2) reading the experimental procedures, and then maybe (3) reading the text if you need clarification on a point or two.  Lots of areas of chemistry (organic, biological, materials science) benefit greatly from visualization because of the fundamental idea that structure determines function.  If you can't visualize a catalyst in 3D, it becomes much more difficult to explain things like stereoselectivity or reaction mechanism.

I visualize math all the time. Geometry, linear algebra and analysis are of course very visual. Probability theory and combinatorics too. Some topics, like group theory, I don't know how to make visual, so I don't learn them :-(

I read Jaynes’ Probability book from cover to cover last year, and most of my understanding of it came visually. This was a big breakthrough for me, because I’d never tried to understand math visually before - I think I thought the graphs in my calculus education were there to explain the equations, and that the equations held the real core meaning. I finally gave up on trying to gain any intuition through equations alone and went visual first when reading Jaynes and this approach gave me 5 or 10 times the value I would have gotten if I tried to understand it algebra-first.

As I have Aphantasia I never visualize when not dreaming.  Before I learned about Aphantasia, I would have been amazed to learn that anyone visualizes while they read.  I  am curious to the extent that visualizing helps learning to see what kind of handicap I faced as a student.  I have a PhD in economics.

I visualize math a lot. Ironically, I don't like geometry, because my visualization is not precise, I don't visualize lines and angles. My visualizations are more like: objects in 3D space; shapes in 2D space; or numbers written on a board.

For example, if you asked me to calculate from memory "464+245", I would keep repeating the numbers mentally (to keep them in the auditory loop), but at the same time I would imagine them written on the board below each other (ones below ones, tens below tens, etc.), and try to add them kinda like I would on paper.

When I think about set theory, I sometimes imagine the sets visually as graphs (small balls connected by strings), with the node representing the set at the top, and the nodes representing its elements below it, connected by lines; etc. Then e.g. the Axiom of Foundation becomes "there is no infinite downwards chain". Then I can use different colors for different types of sets, etc. Similarly, nonstandard integers are horizontal chains of small balls, etc.

I have a picture in my mind, then I read the axioms and translate them to statements about the picture. This sometimes allows me to make intuitive guesses, such as "it is impossible for a picture to be both X and Y" or "a picture that is both X and Y would have to look kinda like this", and then I try to translate the intuition back to the language of mathematical statements. Sometimes it turns out I was wrong, then I try to fix the picture.

It seems to me that the auditory and visual processing have different advantages and disadvantages. If I have to remember the number, the sound of "six - nine - eight" is more reliable than the picture of "698", because the sounds of individual digits are different, while the pictures are similar. On the other hand, the auditory loop is linear, and beyond certain size you have to use pen and paper; while visualization allows you to see 2D or 3D structures and mentally moving and rotating them, and make some statements that are simple in the visual form, but their description in words is clumsy. (It is easier to move your hands and say "rotate like this" than to say "rotate around the vertical axis by 90 degrees clockwise".)

I personally do not visualize whenever learning or reading STEM material. I think mostly in words. Weirdly, I do find that diagrams and graphs help make things clearer for me, but I don't (or sometimes can't) visualize them myself in my mind's eye (even though I understand the concept).

The first layer of internal visual experience I have when reading is a degree of synesthesia (letters have colors). Most of the time I'm not aware that this is happening. It does make recalling writing easier (I sometimes deduce missing letters, words or numbers from the color).

Then there is the "internal blackboard", which I use for equations or formulas. I use conscious effort to make the equation appear as a visual experience (in its written form). I can then manipulate this image as if the individual symbols or symbol groups were physical objects that can move and react with each other. This apparently allows me to solve more complex equations in my head than most mathematicians. (I believe this is a learnable skill.)

Finally, there are the visual experiences that I use to understand concepts. I'm not sure how to describe these, because these certainly aren't actual images that are actually possible. More like structures of shapes, spatial relations and other "sub-visual" experiences. It's not like I can actually visualize an n-dimensional subspace, but it isn't simply a lower-dimensional analogue either. It looks thin, but with a vast inside, in a way that would be contradictory in "normal" visual experience.

Whenever I read about a concept that seems interesting (e.g. Moloch), I pause. Then I take the verbal experience of what I've read, and use it as a guide for some internal thought process to follow. The nature of this process is the creation and manipulation of impossible visual experiences of this kind.

These days my visualization is a lot fainter than it used to be, so faint in fact that sometimes I barely see anything at all, in spite of knowing what I'm (not) seeing. This includes my dreams, and maybe even waking experience (how would I tell?), and I believe this is unnatural. This only seems to have a negative effect on the "internal blackboard", but not on any of the other mechanisms I mentioned.