Yeah,.. I've read through the whole entire Bible and I definitely missed this part. No offense to you or God or the Bible,.. but it sounds like some sort of a science fiction or something.
Well, Moses certainly wrote what was already well known, and did not need to elaborate as it was already written
Genesis 6 moses wrote sons of Elohym took/ married daughters of Adam and went in to them, and giants were born. Moses makes it plain that sons of God did that before the flood and also after that.
Jude, womb brother of Jesus, also wrote to comment about it.
Also, the giants before the flood were taller than after the flood.
Also, some, in the days of Moses, were as tall as oak trees.
Israel killed all the giants in the land after they entered under Joshua, fighting many battles with them until there were none within their boundaries, but there were many in other parts of the globe, and even today, though smaller in stature.
Jesus said they are the tares who will be gathered up by the angels and cast into the lake of fire, with no judgment then, because God the Word gave their doom already, through Enoch the prophet, the seventh from Adam.
If you ask a question but are not willing to do research, then you are just gathering opinions and will be ignorant of the matter in the end.
The real Book of Enoch, chapters 14, 15, tell exactly what they are, and what their judgment is, and why they roam earth until Jesus returns to judge the world.
They are not fallen angels. They are evil spirits on earth when disembodied. Those who walk in the stolen Adam flesh bodies are not human in spirit, but are tares