Those are primarily associated with DMT (the one time I tried it, I too had such an encounter and I didn't know it was a thing until years later).
I'm sure I'll be corrected on this but I think DMT and Psilocybin ultimately affect the same pathways so it's just more evidence that Machine Elves are real! (/s on the real part).
Unknown compound ATM.
I've done plenty of psychonaut adventures in the past but it was only that one experience with DMT that actually gave that experience -- but it was also the only time in which I completely disengaged from local reality.
In chemistry it often only takes a single atom difference to totally and radically change a molecule's properties.
My (extremely limited) understanding is that humans are "hardwired" for pattern recognition (especially faces) -- could there be some sort of ROM equivalent of "this pattern is another human" that gets engaged by the chemical experience?