>>44947
Yeah, it seems to be a weird mixture of Buddhism, Christianity, Taoism/Onmyoudou and so on:
Upon death your yang soul (phantom/hitodama) goes to Higan and awaits judgement by the Yama while your yin soul goes back to earth. If a soul's body gets stolen before reaching Higan it becomes an Onryou and goes to the Underworld. (I assume this means that the yin soul stays in the body until judgement.)
Shiki Eiki is the Yama responsible for Gensokyo, but worlds like Makai and the (real) Moon are probably not under her jurisdiction.
The Yama decides what happens to the soul:
It's either directly reincarnated, sent to Hell for a while to atone for sins or to the Netherworld if it is guiltless. And as soon as it achieves nirvana in any world, it ascends to Heaven.
Interestingly, suicide doesn't appear to be a sin due to the link between the Netherworld and Aokigahara.
I don't think that Shiki Eiki created the worlds she uses, but she can extend them. My theory is that Yamas are assigned to districts and can decide for themselves how to judge souls.
But Buddhism is probably just in control of places with a Buddhist religious majority.
Hakugyokurou seems to be mostly based on Chinese tradition, e.g. the concept of Hun and Po (=Konpaku) and Hakugyokurou being the the place where artists go after death in Chinese mythology. Meikai (=Netherworld) literally means "gloomy world". This seems more like the Shinto underworld Yomi to me.
Anyway, Youmu's species probably exists solely to watch over the souls in the Netherworld. Regardless of the religion in control, they are (like Shinigami) an essential part of afterlife.
Both fear and desire of death are absent in Half-phantoms according to Miko, probably so that they remain impartial.