The Hell-world is made up of six realms stacked on top of each other. From top to bottom, they are: the Sun Realm, War Realm, Ash Realm, Fire Realm, City Realm, and Frost Realm. Each one has a unique biological and magical biome, and many are the homeland of at least one type of demon or infernal creature.
For the most part, the realms are separate but connected. This means that demons are somewhat able to move freely between them with the usage of portals, but portals are not easily accessible and require physical items to cast. In some realms, those physical items are nonrenewable resources and are either extremely rare, or completely impossible to find. In that case, only very proficient mages are able to craft functional portals, and generally traveling between realms while using their services costs a hefty fee, and therefore transports are typically local government-sanctioned or done in large groups in order to split the fee evenly across all parties.
While the distance between realms can seem astronomical, there is some bleeding over of reality and circumstance between the realms, especially those that are neighboring. A volcanic eruption in the Fire Realm can be felt as a quake in the City Realm, just as the endless battle in the War Realm can be physically heard in the quiet areas of the Sun and Ash Realms.