What is the boy and Yonder?
The boy is an alien.Pretty obvious explanation; the boy and Martin to an extent are a race of interdimensional, parasitic extraterrestrials, most likely serving as a Hive Mind who lure couples into Yonder to feed on their attention and life force until they mature and leave Yonder to start the cycle again. They are a short-lived race akin to mayflies hence them maturing rapidly to succeed the other once they die. Yonder itself is a simulation where the child is transported to in order to experience human behavior. It also explains the book that the boy showed Gemma; it was written in their bizarre, alien language.
The entire movie is a dream; Martin ghosted Tom and Gemma.