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.
The "Cuckoo Entities" in the film are members of The G-Man's species
The movie portrays them as appearing similar to businessmen, awkwardly attempting to appear human and mimic human behaviour, similar to the character from the Half-Life series. They also display some level of reality-bending ability, also consistent to the G-Man. The film shows how they reproduce, and takes place before the Black Mesa Incident.- Both G-Man and the salesman have trouble with articulating words and breathing; they take deep breaths once in a while as if they just wanted to simulate breathing for the matter of disguise.
