We are always confused with the giant beasts in the Bible as what kind, big, huge, gigantic humans. But we thought there were some kinds of humans with very big bodies.
We understand from the book of Genesis 6:35 which shows that they are just ordinary humans but they came as angels. It's like a crossbreeding of angels with humans. The Son of God in the Bible also shows that it refers to them as angels and not just humans.
These angels who left the state fall from heaven and they lose their identity. And they lost it against the daughters of men. They started mingling with men. They were chained and kept in prison for judgment. This shows that there was an intrusion into the human realm which caused a lot of crossbreeding.
The giants came into the beings but these giants were extra strong and they dominated. These were just mighty men. They were the ones that caused the most havoc on earth. Especially during the days of Noah where there were so many sins.