According to the Bible, they are fallen angels known as the Sons of man in Ezekiel 28, and since Satan, once known as angel Lucifer rebelled against God and fell from heaven to earth, he took one-third of the angelic host. These demon angels joined the insurrection in Revelation 12 against God. We know the angels that went with Satan as fallen angels who became demons.
Now not all the angels are bound according to Jude verse 6 says they bound the angels that sinned “with everlasting chains.” (1 Peter 5:8) But the angels that sinned against the women in Genesis 6 are bound in chains. The referenced angels in Genesis 6 were “sons of God,” but they sinned against the mortal women, and their disembodied souls became demons. A new race of people known as the Nephilim was the offspring of the Nephilim. And their children became demons and were killed during the Great Flood in the Bible. Read the book of Enoch (1 Enoch 6-8) for more information. There are many topics, but reading the Bible will help you understand the conflict between good and evil. Read Just the Edge of God