i would guess that you are maybe talking to the wrong person because i don't think i have this condition, and also have a fairly abnormal perceptual system with some quirks that apparently most people don't experience (very strong visual snow which is basically always present, and frequent but mild drifting/breathing hallucinations, tracers, etc)
i would suggest replying to irenes' comment on this same post to compare notes with them, since they have afantasia and can probably help you figure out better if you do as well
what i will say is that for us, we conceptualize of our visual system as being made up of multiple modules (and note that this is not entirely a scientific model, this is just how i infer the system to work based on a mixture of reading scientific literature and carefully observing it myself)
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the material body parts that serve as sensors- eyes, ears, skin, etc
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the internal map of the body. this is not necessarily the same thing as how the body is shaped on the material plane- see for example phantom limb experiences, the experience of being trans, otherkin, etc
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our sensory feeds from our sensory body parts. the brain maps sensory feeds onto the body map, approximately where they were received from- thus we (and quite probably, people generally) have the sensation of our visual field usually being located "at" our eyes. when we feel something touch our skin, the sensation of being touched is mapped onto that location in the body map, we feel loud sounds "at" our ears, etc.
but note that you don't actually get to see your raw, unmanipulated visual feed, hear your raw unmanipulated auditory feed, or any of your senses. they are closed off to "you" by your brain, because it needs to process them first (manipulating them in often radical ways) to make it comprehensible to "you"
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the sensory processing chain, an incredibly complicated neurological system located deeper in your brain, where innumerable calculations, adjustments, interpretations, and alterations are performed. my assumption has been that many of my mild perceptual hallucinations are introduced somewhere along here, while some (like my tinnitus, vestibular issues, entopic phenomena) probably are introduced physically at the source of senses.
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the actual place in the brain/nervous system where the experience of sensation is actually ""happening."" we can call this the "qualia place" for short. in reality this might not be one specific central physical place, but it kind of feels like one.
unless they've thought about or read about this a great deal, people generally confuse all of the above mentioned things as being the same thing- because the sensory feed of vision is mapped onto the physical eyes, people imagine their eyes as being like little windows they are looking through. but in reality, the mind is juggling all these components, to produce a very effective illusion. everybody is constantly hallucinating consensus reality together. when your hallucinations differ from those of others, or when you dream, you are probably experiencing those things in the qualia place, rather than in the actual incoming visual feed, but they feel very real and are not necessarily overridable while they're happening
- sensory attention - i.e. what senses (material or imagined) we are devoting attention towards, generally to the detriment of others. if we are listening very closely for something with our material ears, we stop paying attention to what we are seeing with our material eyes. if we are examining an object very closely, we might not notice a lot of the sounds going on around us- a sensory feed can "fade away", because we aren't actively processing it, aren't devoting attention to it.
note that this fading away is not unique to replacing a physical sense with a imagined one and in fact happens frequently for us with our physical senses alone
- our imagined senses (mind's eyes, mind's ears, mind's touch, etc). when we imagine stuff, those imagined sensations go to the qualia place, but generally speaking they aren't as vivid as the feeds from the physical sensory system, unless we devote a lot of sensory attention to them. the more sensory attention we devote to a imagined sense, generally speaking the less sensory attention we have left to devote to our physical senses, and so if we are trying very hard to imagine something, that outside world fades away in response to just not having enough attention left to be experienced.
so in one sense, we are overwriting the visual feed from our eyes, but in another sense, it's not the same thing as experiencing the visual feed from our eyes in the qualia place, when we imagine our headspace, it's not usually as though we are standing there physically, and it doesn't usually feel as though that information is coming from our physical eyes
getting our imagined senses to appear realistic and vivid is difficult, and it is a skill that requires practice. if we haven't done it deliberately in a while, we are worse at it, and the imagery is more faded and shadowy. when we were actively constructing our headspace through deliberate effort, we were practicing almost every day at imagining more and more vividly.
sensory imposition is the skill of using your mind's senses to edit your physical senses' feeds. it works because you are adding this additional information in the qualia place. i can imagine somebody hugging me, and impose it onto my physical touch sensation in the qualia place. it doesn't feel quite like the "real" thing most of the time, but i imagine if i practiced at imposition more i would get better at it and it would feel more "real".
i can edit in some object that is not there in the physical visual feed in the qualia place, but it does not appear as "real" as all the other objects. it requires a lot of sensory attention to maintain, and because i'm now editing my visual feed in the qualia place, everything takes on that shadowy brain feel of an imagined thing, and if i trip up and start paying attention to the physical feed instead, it stops being there. again, i imagine the more i practice at this, the more realistic it might look.
the Window in third person is a kind of middle space between imposition and pure imagination, the sensory information from the physical eyes is being spatially reprocessed within the qualia place by your mind's eye. the processed visual feed contains information about the room you're looking at, and then your mind's eye rotates that around, infers what it would be like from a different camera angle. because this requires an enormous amount of sensory attention to accomplish, your physical visual feed "fades away" due to a lack of sensory attention
when you lose your focus, and stop rendering the world in third-person in the qualia place, the outside world fades back in, because you are no longer distracted from it by playing third person. you once again have sensory attention to devote to your physical visual feed.
(or in general have some form of headspace because i can make one up but its only there when i consciously create it and maintain the though)
our headspace is only "there" when we devote sensory attention to it, though the structure of it is persistent because we spent a lot of time constructing it (and would like to go back to doing that, but it involves rebuilding a habit)
anyways. this is my interpretation of senses. i hope any of that made sense and helped you better understand in turn how your sensory system might work (this model is accurate for me, but it may not be for you! that's for you to decide through observation and experimentation)
