Lucifer (satan) is a seraphim, yes.
the cherubim were the "watchers", discussed at length in the
apochryphal book of enoch.The "demons" of abrahamic religions are the "giant" offspring created by the fallen watchers getting it on with human females that they found sexy. (genesis chapt 2, and first part of enoch.) To make this "work", the watchers altered themselves. in doing so, they ceased being the perfect creations of god, and became corruptions that could not be redeemed. Lucifer was the architect of this, after convincing a good third of the angelic hosts to abandon god, god's creation plan for mankind, and do thier own thing instead. Lucifer was #2 on the totem pole, being the literal right hand man of God. (the other being Michael, who remained loyal during the attempted coup) The angels are not normally physical beings, and are instead "spirit beings." This is why alterations were necessary, and also why they could not be redeemed afterwards. Thier offspring, being a synthesis of fleshly bodies and spirit bodies, had no real place either in heaven or on earth, and "survived" after thier deaths as incorporiated spirit beings, but retaining fleshly appetites. Their fathers were banished (to what I swear sounds like a description of a big metallic asteroid), and imprisoned away from the earth until the end of days, before the time of judgement-- at that time, they will be judged along with what remains of thier children, and the wicked fallen of the human race.
Demons possess humans in order to sate these more carnal apetites/vices, and also out of spite; they hate humans, who are given every chance to succed, where they are/were denied even a single chance, and were doomed the moment they were concieved. These hybrid offspring are called "nephilim", or "fallen ones."
Yay religious studies. Yay.