The Nine are very powerful beings, possibly linked to a power like the darkness. They could have been human or awoken at one point. Petra's dialogue in HoW makes The Nine sound like they are not ones to be messed with. Xùr is their agent, possibly a creation of theirs. He claims to be a trash collector for The Nine, so maybe exotics are just a garbage byproduct of whatever they make

If Hereafter and other stuff are byproducts, I'm going to throttle the Nine for their main products.

Who's to say if some of our exotics are byproducts, and if what The Nine are actually making would even be usable to us