(In response to this post.)
This is awesome.
Think about it: The “other dimensions” the Iplier Egos could come from might actually be alternate universes. So all the characters we see? They look like Mark because they’re that universe’s version of him. So Dr. Iplier is who he would have been if he’d gone onto medicine, Google exists in a pre-Terminator/I, Robot type of world, and so on. (If you’ve seen the Jet Li movie The One, that part my theory will make more sense.)
Darkiplier and Wilford Warfstache would be special cases, as they don’t obey the laws of physics and exist on entirely different planes, not just alternate Earths. Dark’s body is a “puppet” He moves around to interact with humans, and Wilford could be much the same as Him in that respect.
Both beings (Dark and Will) are essentially Lovecraftian monsters who decided to take human form in order to comfortably exist in our world. One is good (kinda?) but just doesn’t understand how humans work (ex: he literally does not understand human mortality), and the other wants to learn more about humanity for His own enigmatic purposes (we don’t know why, exactly, but He’s clearly up to no good).
Wilford might have chosen to mimic Mark because he was the first human Will ever met (albeit adding the trademark mustache to set them apart). Dark likewise must have chosen to take Mark’s form because He thought resembling an attractive Youtuber with a large fanbase would be an effective way to manipulate people (after acting through Mark, first, over a number of years).
That’s just my two cents regarding Mark’s egos. And I’m not familiar enough with Jack’s characters to speak on the nature of their existence(s).