Salamanders have no god, and are kind of proud of that. They revere the sacred pillar of fire, Illiu, and many worship it, but it doesn't walk around or give orders.
Undines have two gods: Ahmn and Mna, representing the male and female undines respectively. They are co-rulers and generally rule without overt conflict between each other.
Illiu the Pillar of Flame is a mana font of great power (+10?). To date, no one has ever attempted to absorb it and lived. In fact, it produces so much heat and flame that it takes Legendary fire adaptation magic just to survive to get close enough to try. The salamander leaders guard the area carefully, lest some foreign god try to reach it.
Outsiders in both Oceanus and Infernus exist at the sufferance of the locals, since it takes constantly maintained magic to keep normal folks alive there. Their guards are quite practiced at counterspelling adaptation spells, since it's the easiest way to kill foreign intruders.
Oceanus is pretty much a water world with an unbreathable atmosphere on top and the civilization is all at the bottom of the ocean.
Infernus is a volcanic wasteland covered in molten lava, with practially no oxygen left in the air and an ambient temperature high enough to kill normal folks.
Both elementals have to be created with Legendary magic of the appropriate sort. They can't breed in the normal way. Salamanders are created by a council of elders who cast these spells near Illiu. Ahmn and Mna take care of it for undines, with Ahmn making "male" ones and Mna making "female" ones. Gender doesn't mean much for elementals and many assume a kind of genderless form instead.
Because of the strength of Infernus, the mana level there is generally +1.
Do they eat anything? They may well have to be like Golems and feed on raw magic. If so, it's a kind of recruiting scheme of magic... they're gifted with innate magic which makes it possible for them to eventually get good enough to make children.
Both kinds send representatives to other realms to employ spies (keeping an eye on other deities who might try to attack them, however impractical that might be), to purchase goods that they can't manufacture, and to receive training in specialized knowledges (particularly magical arts other than their natural one). Both regard elementals who like the other realms as being weird and kind of unnatural.