For demons to exist was according to the permission of God, if God did not permit demons they will not be in existence according to the book of Act 17 vs 24 entails that the creations of demoms was orchestrated by God. Demons are Spirit that we can not see but we can see it's manifestation and they live in different forms .
Demons are manifested to force one to act or do what is not right in the sight of God that's why is important that the Holy Spirit should dwell in someone in order to work against these demons.
In Matthew 8 vs 28-29 made us understand that when demons saw Jesus Christ they started saying that thou Son of God meaning that demons knew who Jesus Christ was and that he had superior power over them for now, there are existence of demon until when they will be bound in hell.
Demon works by possessing a fellow that is the flesh of a fellow and when the demon takes over the fleahof that action or attitude of su h fellow is controlled by that demon or manipulated by the demon.
The only way they can be free is when they are cast out in the name of the Jesus as the sole controller of demons and by being born again with Holy dwelling in such fellow.