How do demons exist and how do they multiply?

Calvary greetings to you all. Trust we are all doing great.

Nothing would exist unless God cause it to exist. The existence of everything depends on the Lord because He makes the world and everything in it.
*Acts 17:24 God that made the world and all things therein, seeing that he is Lord of heaven and earth, dwelleth not in temples made with hands;

When satan was created,he was not satan. His name Lucifer. His name changed when he thrown down from heaven. His ambition made him to an arch enemy of God. He wanted to be as high as God and hence was thrown from a heaven

Demons were not demons,there were not created demon. They became demons because they wanted dominoin
*Jude 1:6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.

Demons are angels that are falling angels that do not follow God but the devil to rebel against God and henc, became satan.
Demons are spirit being. They don't multiply as humans do or produce their own kind.
*Revelation 16:14 For they are the spirits of devils, working miracles, which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.

Demons are angels converted by satan to rebel against God.

Thanks for reading through I come your way, remain bless.

Here is the link to the Bro Eli Soriano teaching on the topic