Aurbis Within the Great Wheel

Though there are various models for Cosmology, they are all derivations of the Great Wheel. While the Great Wheel always exists, there are many configurations of transitional access that permeate it. An example is the World Tree of Yggdrasil or Mount Olympus. The Material Plane surrounds Mount Olympus, yet descend and you will venture to the Lower Planes that have influence upon that material world, and to climb Mount Olympus brings you to the Home of the gods in the Upper Planes.

 Same is true with Yggdrasil with the Prime Material being at the bottom of the trunk. Venturing down to the roots brings someone to Niflheim, and climbing the tree if possible can bring an adventurer to the Upper Planes of the Gods. Not all planes can be reached by these transitions.

In the Forgotten Realms, the World Tree connected all the deities realms to each other, and from there would allow access to the other planes the Deities realm was part. Though these deific Realms were sometimes infinite (and Sometimes not), they were part of other planes throughout the Great Wheel. The World Tree was indeed sentient and it would often object to travellers using it as a simple Transitive Plane.
The Aurbis of Tamriel is similar to the previously mentioned cosmologies. In Forgotten Realms educated inhabitants on the planes knew of the relationship between the Great Wheel and the World Tree. In other worlds, such as those that venerated the Norse Pantheon, the cosmology of the Great Wheel was never revealed to them, and so sages were only aware of the planes for which they had access. The Aurbis of Tamriel is a result of the creation of the Aurbis which may be the perception of a living god’s dream.
Oblivion, is the part of the Great Wheel that encompasses the Lower Planes of the Great Wheel, the Plane of Shadow, the Ethereal, and the Elemental Planes. Aetherius includes the Upper Planes of the Great Wheel, The positive Energy Planes, and the Astral Plane. Sages and Magic users of Tamriel have not developed magic or Portals that can get them to any of the planes of Aetherius, and no magic has been discovered there that allow an individual to directly transfer to another plane or dimension. All planar travel seems to involve rituals that create a portal. When the Pact with Akatosh was made, he sealed off the ability of anyone from the Lower Planes being able to create a Portal to Mundus, and the only way for Daedra to enter the Mundus was via Conjuration summoning spells. When the Oblivion Crisis Occurred, the barrier between Mundus and the Lower Planes was weakened and this allowed Mehrunes Dagon to invade with powerful armies. Martin Septims sacrifice sealed the barriers again and prevented Oblivion gates from opening thus forcing the Daedra to rely on Mortals to opening portals for them through rituals, or shrines.