My theory is 3PO gets lost in the Death Star wreckage, finds Palpatine’s darkside spirit, and he uses the droid as a host until he finds Rey.
As much as I really don’t like that idea, you cannot deny that sounds a lot like what Voldemort did in Harry Potter. After he lost all his powers, he had to survive off of different mediums before being reborn into a new body in the Goblet of Fire. This here sounds like how he shared Quirrell’s body by having his face on the back of his head.