It’s not like a written or verbal list though. I also have no internal voice so that wouldn’t make sense. It’s just like the concept of what I’m thinking of is right there in all its detail. Its extremely spatial - I’m thinking in 3D even if I’m not visualising it.
On the visual side, sometimes if I try hard I can make out an amorphous blob. Mostly colourless, though sometimes it has some abstract colours. Trying to recall actual detailed features is very hard, especially faces.
Occasionally I get memory flashes which are more like actually seeing a photograph in my head, but they last a fraction of a second and can’t be done on demand. Sometimes I have dreams which are more visual. This is how I know that my normal way of thinking isn’t visual.
A simple test I've seen mentioned is, ask someone this: “imagine a car, a fast car, zipping through a windy road… ok? (pause) now, what color was the car you saw?”
If you even need to think about it, you hadn't seen it.
Famously, there's a psychology experiment where a person in a gorilla costume walks through the middle of a scene and beats their chest before walking off the other side of the screen, but people who've been given a challenge of tracking a ball being passed around will completely miss the gorilla. They'll laugh in shock on watching the same video a second time, amazed that they didn't "see" the gorilla on first viewing when their attention was on the ball.
In your simple test, focus is going to be drawn to other components - "fast", "zipping" and "windy" make me pay attention to the curves of the road, the wheels, the trees or cliffs causing the road to wind. The color of the car is irrelevant, so I don't pay attention to it.
I can't tell you what color the car was, but when I watched the gorilla video (without knowing in advance about it) I didn't know a gorilla had walked through the video either.
Consider attempts in this very thread to compare conscious visualization to visualization in dreaming. Someone who isn't in a critical frame of mind or doesn't know about the limitations of vision in dreams and how our brains trick us about dream-sight (or the fairly different limitations of real vision and how our brains also trick us about that, as you mention) may follow a train of thought like, "well, I 'see' just fine in dreams, and my conscious 'mind's eye' is very similar to that, so sure, by the transitive property, I can 'see' about as well when I visualize as I actually see things with my real eyes"
Me, I go "well dream vision for approximately everyone is total shit but with a layer of trickery on top, and my 'inner eye' is similar to that except with the trickery dialed way down so I can tell where the seams are and if I try I can be aware of when I've just invented some detail that was 'always there' but actually wasn't a moment earlier and I can tell that I'm not actually seeing with my eyes (unlike a dream, where I think I'm 'seeing'), so yeah those two are pretty close for me, and the ways in which they differ are basically just how much my brain's lying to me so arguably aren't 'real' differences anyway, but both are entirely unlike actually seeing, so no, I don't 'see' when I visualize the same way as I 'see' with my eyes, though it is close to how I 'see' in a dream except I'm less-fooled about how bad it is"
... and I propose that these two responses could come from people with identical actual capacity for mental visualization.
When one of the former meet the latter, it might end in the latter thinking they have aphantasia or at least lean farther that direction, without any difference in their actual experience of or capacity for visualization.
....
I've seen a supposed set of autism test questions (I don't know if they're really used in autism diagnostics) that include something like "would you rather go to a party, or stay home and read a book?" and supposedly the "autistic" indicator is asking follow up questions or excessive hesitation. Meanwhile I'm very sure you could find people who instantly answered "go to a party" but actually choose that far less often when presented with the real choice involving those two things (necessarily with a lot more details and context filled in). I don't think they're lying or deceiving themselves! I think they're regarding the question very differently from how some others do. I think something similar is going on here, with two "tribes" with different perspectives on the question itself trying to communicate and talking right past one another, leading to much confusion.
(Meanwhile, I do think it's entirely possible aphantasia is real, I just also strongly suspect a lot of the people who've been led, by online discussion, to believe they're far from the median in this regard, actually aren't)
This is very much a real thing, but largely goes unnoticed because it doesn’t really affect anything, except for people going about their lives thinking that the word ‘visualise’ is a metaphor.