So many questions remain. What are Death Gods and where do they reside before being summoned? What created them and for what reason? What the hell was Kaguya's ultimate plan? We only got to see her get as far as beginning to create a new dimension, but for what? Why an army of Zetsu? Was Jashin the source of all this, did he even exist otherwise where did Hidan get his abilities from? How does one "invent" a jutsu? Was Kaguya really an alien? How does that tie in with her plans? What were those castles in the background of Kaguya's dimensions?
Someone help me out here.
This will all be speculation, but here I go.
The death gods are simply an existence, no one created them. Mortals simply found a way to temporarily summon them. In regards to where they reside or how they live, I'd imagine they are in another 'dimension' and simply reside without much consciousness, if any. By that I mean they don't live like a human does, more likely a constant state of meditation or even comatose.
Kaguya seemed to only seek power and domination. Her Zetsu army was a way to channel more power to herself while subduing adversaries and creating a way to instill fear to the masses. Kaguya was most likely an alien. She could have been cast out as a traitor, extremist or a failure. Her domination of Earth could have been ordered or could have been an attempt to conquer a planet after loosing her place in hers.
Hidan's abilities are far less predictable. Considering there is no one else like him, it could be one of 2 things. First those abilities could have been unique to him and he found purpose for himself believing in a higher power. If he did actually acquire his abilities through supernatural methods, I'd believe Jashin was like another deathgod that Hidan was able to channel power from in some way.
Jutsu are simply ways to manipulate chakra. Once you know your affinity and how to channel chakra, you can create any jutsu that is physically possible. Kakashi created Chidori, their creation is only limited by imagination and physical impossibility.