Incidentally, there are two general explanations for Nephilim showing up later in scripture (I mean while still fitting the pre-Noah story).
#1.) The rebel angels did it again, just not as extensively for whatever reasons.
#2.) At least one of Noah’s sons actually married a Nephilim woman! Consequently (what we would call) the genetics would keep popping back up in descendants on occasion. (Artistically this is my favorite theory, and I’d be tickled if it showed up in the upcoming super-gonzo Noah movie with Russel Crowe. Note that it relies on a detail of human gestation which the scriptures barely even hint at, the contribution of the woman to the baby – that’s an important detail of the humanity of Christ in the Incarnation for example, but most of the time the scriptures seem to assume the women only host the material from the men and contribute no material themselves.)
Also, though I don’t think this changes the weight in regard to any theories discussed so far I guess, a “docetic” anything wouldn’t have any real flesh or blood, borrowed from someone else or otherwise. That’s the point, the appearance is only an appearance. (Notice however that on typical ancient ideas of human gestation, there could be no virgin birth per se, only at most an appearance of such.)
The various theories about how the “sons of god” (if they weren’t what we would call advanced Adamic humans to begin with but rather rebel spirits of a basically different species) generated the Nephilim can get rather… interesting. One theory, which I don’t personally subscribe to (being rather positively agnostic about what exactly happened anyway), is that they took bodies made of clay, enticed/collected seed from human men as (what would later be called) succubi, made alterations to it internally, and then shifted the form of their bodies to impregnate human women as (what would later be called) incubi. In some versions of that theory, the point was to create actually organic bodies for their spirits to inhabit, since they couldn’t do that outright for whatever reason (or for whatever reason didn’t want to, details tend to be vague on this point), and so they birthed themselves into the natural world this way. However, they could now be naturally killed, BUT the bodies so created were so strong due to various design mutations (killing the women in childbirth is usually part of the story here), that they could only be naturally killed by a major natural catastrophe. (Why God would do that instead of killing them more directly tends to be rather vague again.)
On this class of theory, the incident explains why God and His prophets are so specially against religious modes which involve having sex with deities, even figuratively through human partners. Sometimes this includes an extended explanation that the whole affair (in multiple senses of that word ) dishonors the coming incarnation and virgin birth, which on some versions of the theory was a (or the) main reason why the rebel spirits did it in the first place. Another version of the theory for the rebel’s rationale(s,) is that it’s connected to Eve’s interpretation of God’s promise that eventually YHWH would be born of a woman (since she names one of her children something that translates to “YHWH is here”).
This theory is also one of a somewhat broader category of theories, including some completely naturalistic ones, claiming an overlap here with popular pagan demigod stories being criticized by Jewish religion. i.e. yeah we’ve heard about Herakles and Perseus etc., and we don’t deny that that kind of thing happens, BUT IT IS VERY MUCH NOT A GOOD THING! – and we shouldn’t worship them religiously, or even respect them at all.
Like I said, I don’t subscribe to a particular theory about what actually happened (and I’ve picked up other ones in various places over the years), but I do think the general scriptural gist across the OT and NT is that at least some rebel spirits who weren’t themselves technically human species, died in a giant water catastrophe, and their spirits are now imprisoned as a result though sometimes they manage to temporarily escape and make trouble before being returned to the prison, which is why the oceans represent and maybe even embody that situation. But then also other rebel spirits haven’t gone through that (perhaps yet) and are either imprisoned by a different method corresponding to what we’d call the atmosphere, or haven’t actually been imprisoned yet even though they still have a difficult time (and maybe increasing difficulties over time for various reasons involving God’s war against them) affecting the world directly.
Beyond that gist, the scriptural details seem sketchy to me. And I don’t claim to know for sure what it actually means or represents, although currently I agree there are rebel spirits trying to affect the natural world, including through corruption of human (and other species) gestation, but they’re having an increasingly difficult time doing so due to various things God has been doing mostly through (more-or-less) loyal agents of His.
I allow I could be wrong to some or any degree about that, and I try not to hang anything on it, and to keep an open mind about further details even along that line. But I have also found it’s important to notice at least that much of the story going on (in various modes of testimony) throughout the scriptures, because that general story (for whatever reason) seems connected to portions of scripture talking about universal salvation of sinners from sin (and to various explanations about human suffering).