Visualization is an extistential concept referring to how our brains think about things that are not currently happening, such as movements we're not doing physically or sights we're not currently seeing. Visualization is necessary for our brain to problem solve, remember things, plan things, and learn inbetween tasks. It's most heavily used when we are sleeping, but we can also utilize it to varying degrees when awake.
Visualizations occur on various cognitive layers of thought in the mind. Not only are there layers for different senses, but these layers can be further divided by the specifics of those senses (such as color vs value vs form in vision) and whether it is conceptual, literal, or somewhere inbetween (as in, are you literally seeing that red apple or are you just thinking of the concept of one). Most tasks only require conceptual visualization.
There is also the issue of whether or not someone is actually metacognitively (https://lib.hol.ink/index.php?topic=42.0) aware enough to identify how conceptual vs literal their visualizations are, as the brain auto-corrects conceptual visualizations to seem literal by default for most people. Those who don't have that auto-correct tend to self-identify as "aphantasic," even if they're actually experiencing the same level of visualization as those who don't identify that way, and are simply more aware of the fact that it's not an actual image.
Aphantasia refers to being incapable of literal visualization. Barring serious brain injuries or surgeries that disrupt thought and sensory processing, being fully incapable of literal visualization is extremely rare. Temporary disruptions from psychoactive medications are more common.
Otherwise, using these more advanced cognitive layers is a trainable skill. Some individuals have more or less starting ability in doing so, depending on subtle factors like the way they daydreamed as children and such, while others will need to train from scratch as an adult. I don't normally recommend this training, because it's extremely time consuming, and conceptual visualization is sufficient for reaping the benefits of visualization exercises. Despite being trained in literal visualization, I almost never use it in practice, nor do most artists I know of. It's more of a fun party trick for mental amusement unless it comes so naturally to you that it's your default mode.
It's worth noting that these cognitive layers aren't a set-in-stone thing everyone has the same versions of. They're something you develop over time in response to your personal needs, so everyone's presentation of them is a bit different. And again, though training, you can add duplicates or new layers to manage different types or overlapping visuals. Like moving from visualizing one image vs multiple images on different planes, or from having one layer for sound vs multiple for different types of sounds (music vs speech etc). Juggling those multiple layers at the same time is its own subskill, as well.
A fun exercise my friends and I used to do in elementary school, for example, was to try to think of two different sentences simultaneously, speak a third, and write a fourth. It's very difficult! The feeling is a bit like learning to play the drums or piano, where each hand needs to operate on its own independent layer of thought accessing the same music sheet. Our natural instinct is to run them on the same layer and thus perform symmetrical or mirrored actions, but those actions are of course unsuitable to music, where we need a wider variety of sounds and beats in a single song.