The “Sons of God” in Genesis 6 are angels and when they took human wives their offspring were giants that we call nephilim; this is how the seed of man was corrupted, God had to destroy corrupted man’s seed in the flood. Verse 9 states that Noah had clean genealogy.
Satan (who is real and walks among us) knew where the Promised Land was and had 400 years to populate it with nephilim, fallen angels bred with the human women, and giants (nephilim) were the result of these unions. Numbers 13:33 says: “There we saw giants (the descendants of Anak came from the giants); and we were like grasshoppers in our own sight, and so we were in their sight.”
The Israelites rebelled because of the fear of the giants and spent 40 years wandering in the wilderness. When Joshua did finally cross into the Promised Land he was told by God to destroy all the people (men, women, and children and even livestock) because the human genetic gene pool had been contaminated by Satan’s minions. This was done to keep the human seed from being corrupted with the seed of Satan's minions.