My current theory is that these other realities we visit only exist while we are there. In a sense, we bring them to reality for the time of our voyage. From this it follows that these realities may be anything that the mind allows to exist (they are manifestations of our minds).
I think these realities (or the laws determining their possible structures) are somehow encoded into us - as latent information - and we have a built-in mechanism - imagination - which can be used to bring this dormant information, these "memories" into actuality. Perhaps God created us with these latent codes built into our DNA, waiting for psychedelics to activate them. And when people activate these codes on a large scale, they transform material reality according to the "directions" of this informational DNA (or rather its manifestation as experience), thereby helping God bring creation to a completion: the transmutation of the immaterial into the material world.