Divinity 2 used Gamebryo and had a lot in common with Oblivion (and in fact I did a fairly straight copy of some of my Oblivion models to D2... well, ish). The current engine I think (but may well be mistaken) started out with Dragon Commander, got a major rewrite for Original Sin, then another one for Original Sin II, and apparently an even bigger one for BG3. Though there're enough similarities to facilitate modding, so e.g. Norbyte quickly adapted his toolset.
I guess what Larian releases will largely be down to the licensing agreements with the various third-party bits of which there'll be an assortment, though I'm not privy to the details. But I guess that's the same deal with pretty much any studio that encourages modding...
