Most Regio are closest to something like the Upside Down. They are recognizable but changed somehow.
A common means that forms a Regio is when a more powerful aura of a different Realm compresses them until they 'bubble' out of the mundane Realm to preserve themselves. Their presence on the mundane will then slowly collapse while it is in its original form in the Regio. So you might find the ruins of an ancient pagan temple (Faerie) in a city (Divine), mostly cannibalized for materials and with new homes build on top of it. But in the Regio the temple is whole and in all its former glory.
Another means of forming (or expanding) a Regio is through activities which cause a surge in its power and potentially cause it to 'crack' or 'fracture'. You can actually find rules for this happening to Magic Aura in RoP:M. It can be done through a great increase in the magical activity or through many different spells, though no matter what it is up to chance. These however are very unpredictable in their formation since somethings movie into the new Regio while others stay on the Mundane. If it happens to a Covenant it might take all of their defenses into the Regio and leave the Magi's labs in the Mundane for example.
Since I have played for half a decade in a Saga that a large portion of the history and some of the game play was related to manipulating a Magic Regio, I made a post talking about some affects of the rules in RoP:M about a year ago.