Planes are interesting thing...just my thought from planes...
There can be as many planes of existence as GM wants but...
I think that demons come from 10 different planes. Every plane goes further from "known" worlid. Every plane is different, demons on planes are different. Planes are not necessarily deserted and hot places (as we see in movies and in books). Those places where things are not necessarily working same way as in normal world. For example the first plane, where so called lesser demons live, might be twisted image from real world. No real civilization, but remains of that are there. Maybe there is a place where unfortunate person keep their base...first plane is terrifying place, but still somehow manageable.
Further the person (caster?) goes in planes, more twisted the things are. Demons grow stronger, get more organized and at 9th plane. There are well organized fortresses. Plane might consist from several floating isles...what ever GM wants. Point is that more the higher plane number, more twisted and dangerous plane is for coming person and for demon too.
Then elementals...those are not living planes actually. Those are spirits, live in spirit world and are partially in "known" world. Those will materialize to this plane when forced, but are kinder in nature than demons, why? Those part of known world, but demons are not. Demons live at other planes, which are their home. Demon's basic nature is twisted, mean and evil...why? Their natural hatred and lust of power makes them that, but also the planes where those live or be, twist the mind bit by bit. Well...there are chaos...chaos is the most dangerous and same time it is not. Only thing why chaos is counted as evil, is, because it uncontrollable and unpredictable and works differently than any mind. It can be figured little bit like...
Lets take some number of persons. Every person thinks different way...then just put those thoughts to same picture and thats it. Hard to describe, hard to organize and hard to control. Demons are the beings that live on chaos plane. There are only one chaos plane. There are no levels like in normal planes. Everything is mixed up there, it covers larger areas, it is between and around the other planes. It is the most dangerous place for any being to enter, it might go well, but suddenly it starts to rain yellow water from the earth, which is sky where windblows from all directions and road seems to continue all the time, but it ends without warning. Normal mind is not able to handle that world. Demons or beings who are from that plane, don't be there. Those are part of it...not from it.
Then some wise guy asks...what are the elemental demons...twisted elemetal spirits or demons who have taken control of the elemental.
Where souls go? That is another series of planes. If demon or some evil entity takes the soul, it goes to plane where that entity came from. If it is evil soul...or good soul...it goes to their gods plane. Bit like heaven in this world is thought to be...those souls might become champions or other good entities. If soul went to other plane, then negotiations to return it need to be done with the lord of that plane.
Where demons come from? Demons can be the souls of twisted, cunning, mean and evil beings of the known world. Those might be the evil spirits...
Well..only some of my thoughts...maybe that helps :-)