It is hard to explain it because the Bible doesn't give lots of details and DNA is sooo complicated. The Bible says that the "sons of God" came down to earth and had intercourse with women. This would mean they materialized bodies. I don't think they were or had two natures. As angels, being very powerful and way smarter than humans, they could just easily materialize themselves, which they did. In fact, angels HAVE to materialize in order to be seen by humans. Those angels that were messengers of God and appeared to Abraham, Jacob and others materialized only for a short time. They even ate. Also, remember that the angel that conteded with Jacob had to be materialized in order for Jacob to see him and grapple with him. But the angel asked Jacob to let him go because it was almost morning. So, it maybe that angels have the ability to materialize for our benefit e.g. so we can see them, but that they're not supposed to stay in that form for long, possible so they don't fall into the temptation that the demons did e.g. wanting to breed with human women.
The nephilim weren't necessary giants. I'm sure some of them were or many of them. But they were also strong. Their differences didn't have to be physical. Also, the nephilim were hybrids because two different "natures" were breeding. And it could be that as a result of that, mutations were created in the offspring. The Bible doesn't say that the demons were the nephilim. The nephilim were the sons of angels and humans so it wasn't them that when the flood happened, went back to heaven. The nephilim died, all of them, when the Flood happened. It was the angels who de-materialized themselves - got rid of their human bodies - in order for them to go back to heaven.
It must've been some mutation that caused the nephilim to be giants or some of them but they were mostly known for their super-strength. Goliath was not a hybrid yet he was taller than "normal". So, it was also probably some mutationn in the DNA. It doesn't point to an evolutionary trait. Also, the Amalekites were big in stature.
justahuman - but super nonetheless