Since the Bible is silent on most of this, who can say?
One of the points I was trying to make was we don't know how manr Nephilim there were. The fewer the number, the less likely it is any of their fossils survived for us to find.
The scriptures do not mention anything about their DNA. There is no evidence to support that Nephilim DNA was any different than human. There is no evidence to support the supposition that the Nephilim were genetically different in any way. In fact, the Bible does not even say they were hybrids. That's an assumption. It might be a logical assumption, but we lack sufficient evidence to support a conclusion.
We don't know how the demons pulled off being able to mate with human women and produce offspring. If those offspring, the Nephilim, were also fertile and capable of reproduction, then they would be genetically indistinguishable from natural humans.
In regards the supposed Nephilim being a separate species, this is evolutionary propaganda. It's a twisting of the facts to support their god-dishonoring theory. Humans are one species - human. Or, if you want to use the biblical description, we are the same kind. The genetic differences between groups of humans do not create separate species. They are variations within the species. Just like dogs. Terriers and dobermans are both the same species - dogs. The genetic differences may result in variety of appearance and could pose danger in reproduction, but they are all still dogs.
These ideas that Neanderthals were a separate species that lost the evolutionary lottery and died out is fantasy. These scientists have latched onto a set of unique genetic characteristics and decided this is evidence of a separate species. This is the same thinking that fuels racism and the separation of humans into distinct groups based on a unique combination of genetics and ancestral place of birth.