What do you think is both the origins(s?) of ghost Pokemon and how they’re unified by a common theme; I. E. How some are explicitly defined as spirits of dead humans, while others are merely natural creatures with ghost-like powers, Vs possessed objects given life through other forces, and how it all ties into the Pokemon world.

Welllllllllll I think we have decent reason to believe that Pokémon’s creators imagine a sort of “spirit world” that exists apart from the material world, and that Ghost Pokémon are all in some way “touched” by that plane, but don’t necessarily all have the same relationship with it.   Maybe some of them are from there originally, while others were once normal creatures that have been altered by exposure to it, or have developed the ability to access it as a source of power (which might also be a thing humans can do in the Pokémon world, as channellers or mediums, or through whatever “ancient science” was used to create Golurk).  Beyond that… I don’t know, and I think that might be kind of the point?  Like, I think the actual real answer to this question might be “there are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy” – that is, there are some things in the universe that just are mysterious, and you can’t logic them out or determine the answer experimentally.  That’s not because we’ve missed something or because the lore is poorly thought out; it’s actually the point, because it’s meant as a comment on the limits of scientific thinking (which… well, to be honest I don’t think Pokémon has a very well-formed idea of how science might work in a fantasy world, and the writers need to read Harry Potter and the Methods of Rationality, but this is neither the time nor place).  The spirit world doesn’t have consistent rules and different Pokémon relate to it in different ways, because if we could understand it, then that would defeat the purpose.  Moreover, many Ghost Pokémon have powers of illusion and a reputation for deception and trickery; they have the means and the desire to obfuscate the issue.