Prelude:
Lower Realm
Upper Realm
Immortal Realm----->> Connected with the Void and other universes.
Eternal Realm: Above every realm. The Highest Realm ruled by the Imperial God Family.
(WARNING: HEAVY SPOILERS!!)
Current chapters:
In the center of the central Universe (connected with the Immortal Realm, It's the place Rex Protected.), where there are severe restrictions imposed on the location prohibiting severe use of magic or cultivation. Hence the modern scientific setting in the current story.