In the vast majority of science fiction and fantasy that uses portals, the portals are a kind of two dimensional object or a long tunnel. Often, if a portal is a free standing door, those on the 'wrong side' of the portal can't even see it.
I study 3-manifold theory in mathematics, which is the study of possible shapes of universes. In math, two-dimensional planes don't make good portals. What we do instead is basically take spherical portals, one in each world, where going through one sphere takes you out in the corresponding points of the other sphere.
This would look very different from most portals in fiction. Essentially, as you approach a portal world A, you would see what looks like a large spherical mirror, except instead of reflecting your world, you would see another world, world B. As you approach the sphere, you experience no sudden transition and can't even tell when you cross the line, but world B appears larger and larger as world A appears smaller.
Once you leave the portal in world B, you would turn around and see behind you what looks like another spherical mirror I which world A is reflected. There is no tunnel, and no sharp transition.
Are there any instance in fiction where such a portal is used? I seem to remember something like this in the very end of the Chronicles of Narnia, where they go to the Garden of Eden and find another world inside of it, but I'm not sure.