My only question on the Nephilim is why didn’t the flood kill all of them off?
That's a tough one, but there are some theories about that. I think the "multiple incursion" explanation is the most likely.
There’s a theory that Ham’s wife was already infected with Nephilim DNA on the Ark.
Another is that “The Watchers” came & picked up where the fallen angels sent to Tartarus left off.
Pinging Mark17 to fill in any blanks or make corrections to the question.
How do you know it didn’t. The fallen Angels were not destroyed and their mating with human females is the origen of the Nephilim. Perhaps the bad Angels repeated their mischief.