Yep, I mean, is a bit tricky, like an optical illusion that requires you to focus in the center, but if i focus enough, I can, imagining a whole person with all their details and overlaying it with my POV is quite hard and usually requires way too much focus, but just overlaying the image of a person over the same person in front of you is easy, specially if you are just changing the body and hands and face are already visible, thus not needing change.
Is a bit weird to explain, is not like an allucination in the sense that it literally replaces your normal vision, is like overlaying another vision on top of your vision, it requires focus and you can tell the imaginary overlay is distinct from your actual visual stimuli but like with some optical illusions you can force your brain to just accept it and you see people nude, even if the nude models are not very detailed.
The easiest stuff to overlay are ruler thingies tho, for example to extime distance you imagine a line similar to the letter "I" with a base on top and at the bottom of your reference and then you can copy paste a bunch of times and count them it to measure stuff roughly. That is the most common day to day use for overlay imagination I use, the imagining people nude is not exactly a everyday stuff. When I was a kiddo I used to use this to play with things that are not toys as if they were, for example I could pick up a 2L bottle, overlay the image of a rocket and play with it, same with a lot of other stuff that had somewhat similar shapes to things interesting for a kid, I still can do it, sometimes do in the bathroom while taking a very long shit.
For me this is normal and I tough most people were like that, like... the phrase about imagining people being nude is there for a reason... and kids do play with random stuff as if they were other things, people look at stuff and use it as a reference to visually measure stuff even if it is rough.
I mean, there is not that much difference between imagining inside the black void and outside, you are just moving it, the only thing is that outside it is fighting with your actual visual stimuly for attention so it requires a bit of extra focus or to anchor it to something similar, you know the visual illusion were there are pink spots but when you look at the center a green spot appear and eats all the pink spots until it is blank but the moment you ever so slightly unfocus from the center the pink appear again, it feels like that, the illusion is named "Lilac chaser" for reference, it feels kinda like that in the sense that if you lose focus, the effect disappears but if you focus yo can keep it?
IDK, as I said, I thought this was normal, so it is quite hard to explain? is not like imagining stuff in the black void, there one can easy mount a movie if they want, a crappy movie usually, but its possible, outside not so much, but it is still possible to imagine stuff there, just harder.