Fascinating thread!
It never occurred to me to describe the inner world! But then a very long time ago we were told we must always keep it a secret so we have (in the real world)
I've no idea if other alters (within our system) experience it the same as far as I know they only come out to be the 'social front' when needed for social outings/appointments. Their jobs were usually to live in the outside world and act as a filter for 'the original self' who is autistic with severe sensory processing issues. OS could only function in the real world living 'through alters', especially when social expectations were high (to attend school, college, work etc like normal/neuro-typical people)
I seem to have the richest imagination of all our adult alters but tend to be heavily influenced by the few films I did get to see from the real world and got totally obsessed with. Narnia was one of them I absolutely loved the film so created my own Narnia on the inside while others were 'fronting'.
Avatar (the movie with big blue people) was also a favourite that quickly turned into an obsession when the body (IRL) developed physical issues and I couldn't cope with all the physical contact involved (during hospitalisation and rehab) I retired to the inside and 'Maddie' evolved to be the new 'front'.
I then lived 'inside' with the obsessive fantasy that one day I'd wake up/get out of the oxygen chamber thing and be able to walk again in the real world. I lived inside as 'Jake' able to run through my jungle on the inside but dreaming one day I'd be able to come back to the real world and achieve the same.
Unfortunately even 15+ yrs later I've been unable to achieve this in the real world as the body now also has arthritis on top of the original condition the body was struck down with. I had to choose whether to spend 'my life' locked in fantasy only or decide whether I wanted some kind of place in the real world now we live a life less social and one I'm more able to cope with.
I can still fly round the woods and fields with a creature by my side but on a mobility scooter with a dog, rather than on foot through the trees like some kind of Tarzan!

or I can become Link on his horse or climbing mountains via the magic of video games! ...of course there's always Narnia or Pandora still there on the inside if I need a break from reality!
Kit