The giants/Nephilim/etc are not of semi-divine origin (see commentary in Challoner or Original Douay-Rheims). Sons of men = descendants of Cain. Sons of God = children of Seth. They result from the intermarriage between descendants of Cain and Seth.
You are probably aware of the following, but on the off chance you’re not, I’ll mention it anyway in case you might find it helpful to your study.
A good rule of thumb when looking for connections in the Bible is to start with what I call the “Rule of Four” (I don’t know the real name, perhaps an actual Bible Scholar can chime in here).
Things tend to happen in sets of four in the Bible (not always, but more than enough to make the rule pretty useful), as to why this is, that’s for another post.
But things happen not just in sets of four, but in particular sets of four that are organized in a particular way. Of the set of four: 3 things will be similar and the 4th will be related but dissimilar. Of the three similar things, two will be more like each other than the third.
Let’s take the Story of Noe for some examples of this: He lets off four birds (there is our set of four) one is a Crow or Raven the other four are Doves (there are our three similar things, one dissimilar). Of the three doves two return (one with a branch) and the third doesn’t (our two more similar than the other). There are four men on the Ark: Noah and his three sons (Cham, Shem, Japeth) (three similar one different but related), of the three sons, two cover his nakedness and the third doesn’t.). It’s always an enjoyable read to go through a passage in the Bible and see where the rule applies.
With respect to the Noah story generally: **one could use the rule to compare either the Four Covenants God made with Israel in the Old Testament (that were replaced by Christ’s New Covenant). God’s covenants in the OT were with Noah, Abraham, Moses and David. **Noah’s covenant was a promise to all people (not just the Israelites) while the other three were specific to the Hebrew nation. So there is our dissimilar covenant. As between Abraham, Moses and David, the Covenants of Abraham and Moses seem more similar to each other to me (do these things: circumcision and Mosaic Law) than that with David (don’t do this thing) but perhaps other folks have a different and possibly better opinion on that.
Alternatively you can look at the Four “Adams” in the Bible: Adam, Noah, Abraham, Christ. Abraham is only the “Adam” to one people (the Hebrew nation) so is the similar but dissimilar one. Adam and Noah are more like each other, each with three sons, one of whom goes astray, and Christ is similar to Adam and Noah in being an “Adam” to all people, but dissimilar (for all the reasons we are aware of).
Anyway, just some thoughts on a Saturday morning that will hopefully offer some small insight. If it doesn’t, well hopefully it didn’t take too long to read

I find the rule of four useful but I will be the first to admit that like any “tool” of biblical interpretation it has many limitations, some of which are probably on display in this post.
Yours in Christ,
Trevor