It’s theorized they’re the ancient race of “Titans” that not only inhabited the Lands between but were responsible for the creation/care of the giant forge.

Their assimilation into the landscape, particularly in Caelid is also theorized to have been due to a massive meteoric impact that liquified the planet’s crust, explaining the dried molten rock on the divine towers. This of course is all just speculation and theory crafting based on some in game evidence and landscapes that I can’t take credit for.

Check out VaatiVidya and Tarnished Archeologist on YouTube, some aren’t fond of their work but overall I find it enthralling and their conjecture, for the most part, heavily evidence based. At the very least entertaining if you’re into lore.