Planet Sharitarn is a Counter Earth, heavily inspired by John Norman's planet Gor. However, unlike Gor, Sharitarn do has ostensible magic. It is fantasy, not science fiction. Even if you will find some light drops of sci-fi, in small things. Like the sky, Sun, Moon, that sort of inconsequential detail.
The Vortex is a phenomenon that transports things; talking and non-talking, living ones included; from Parallel Universes to the Universe of Sharitarn; and all Parallel Universes touched by the Vortex are identical in Astronomic scale to the Universe of Sharitarn. Only the Parallel Planet correspondent to Sharitarn is affected.
Same number, sizes and shapes of celestial bodies, in the same relative positions to each other: that is all I mean by "identical in Astronomic scale". Because some Universes do have physical elements and constants significantly different from those of Sharitarn. Others_like Earth Universe_ are mostly identical to Sharitarn in therms of basic laws of Nature.
Earthlings became 'involuntary immigrants' on Sharitarn for first time around the year 1500 of Gregorian calendar, give or take a few decades. It is a one way trip.