>>280556538It was all caused by their connections to the other side/paradise, they had an obsession or a hangup that was causing them to desire suicide, which they managed to separate from, and in doing so created a gate. Whatshername with the tumor causing her anxiety, the other girl with the reflection and her scarring, guy and his doppelganger/shadow we didn't really see the resolution of but it still ended with his shadow being separated from him and turning into a gate the same as Pii-tan's stomach and the rest.
The robot is the one that doesn't really fit since it doesn't have anything to do with stomachs and Pii-tan never really separated from it, he just kinda grew a gate on his own and then exploded, but I guess maybe that's why he exploded
Anyway they either embraced or fixed their suicidal issues and literally and figuratively passed beyond death to reach enlightenment. Ito isn't really the kind of author that maps out things 1 to 1 to higher concepts like that anyway, like Pii-tan probably had a robot because he wanted to give a robot a soul and make it explore the afterlife before he even had any of the rest of the ideas for the story but it's one of those kinda packages of concepts that often come together, ideas that are close together in ideaspace.