Where does all the stuff come from?
It is important to note that hell does have an ecology of sorts, but it's not one run on plants and animals. When you want to make something - anything - in hell, you will need to either steal the material from somewhere in the material planes or make it out of tougher stuff yourself - preferably, out of souls. Some rare other materials like alchemical substantia prima can be used, too, but it's quite likely that the hellhound over there is composed of mortal souls. And you don't even want to ask about the chair you're sitting on. Sure, most likely it's from actual wood, but there's always a possibility ..
What exactly are demons, anyway?
While demons are hard to permanently kill on other planes, hell itself is one of the few places where they can be gotten rid of simply by means of stabbing them with sufficient badwill. That proposedly holds true even for the demon lords and emperors who rule this place, but nobody has managed to test that hypothesis yet. Lesser demons, however, ceaselessly attempt to get rid of their rivals for good, and while those rivals are here, they have reasonable chances of managing that. Oh, by the way - you can meet just about any kind of demon in Inferus (don't ask why the name of a neutral spot is so blantantly latin, it's a political thing), but other regions are, of course, populated by specific, culturally appropriate sub-species.
Demons are bad people. That sounds trivial, but I mean something quite specific by it: First, demons are people - that means they have indivual emotions, memories, intentions and plans, can disagree with one another and are predictable when you know them. Second, from a non-demonic point of view, they are bad - killing, maiming, torturing doesn't strike them as bad in any way, and they will happily engage in it when they feel like it. In fact, most of them want to do that not just because of psychology, but also for a quite practical reason:
Why do demons (and angels) care about souls?
People go to hell mostly because demonic psychopomps snatched their soul when they had the opportunity or because the gods decided that's where they should go after death, but some rituals and natural phenomena exist that can do the same thing. It would be far fetched to say that torturing the damned is the main business of hell, it's more of a means to an end (as well as just hilarious to demons): Most dead people who go here will be transformed into demons themselves at some point as a natural property of the place. Pain and suffering accelerate the process and can influence the outcome to a degree, and as the newborn demon won't hold anything in common with the soul that spawned it, every demon ruler tries to get a hold of mortal souls to reinforce his troops.
How do I get around in hell?
The Five Rivers
Each and every hell, no matter how much of a wasteland it may be, has at least one stream of one of the five fluids (ice, fire, poison, mud, oblivion) running through it - by those streams, all of the hells are connected through a system of waterways only known as the five rivers. Navigating these hazardous, mist-covered rivers requires long training and experience, and the ferrymen usually extort horrendous fees to any passenger they don't transport by order of their lord. To compensate for that, the Rivers are usually peaceful enough - only a daring few pirates have managed to unlock the secrets of navigating safely, and the water does not seem to have any natural life in it. Travel by the rivers is usually quite safe and moderately fast, compared to ..
The Green Road
Not every hell has vegetation, but it appears that they all have at least some remains of it - petrified trees, burned wood, or even gigantic toadstools. The Green Road is a universal web of mystical roads that connects certain patches in these regions to the Devil Jungle, and through that, to each other. Paved with malachite, it is usually not hard to recognize when found, but demons avoid it, preferring to travel the five rivers instead - the waters of hell are treacherous and will likely kill you if you fall in, but, save for the occasional pirate, peaceful enough. The Green Road, on the other hand, is fraught with dangers lethal even to demons - murderous blood-drinking plants, bandit bands composed of murderous halfbreeds and rouge demons, and the native wildlife all serve to dissuade travellers from seeking passage here, and will happily feast on those who try. Still, there are some who dare this way - some even manage to survive. Empress Itokue has been secretly trying to establish dominion over the Devil Jungle in the past centuries, with mixed success at best.
Demon realms of note:
Yomi, Realm of the Toad Empress
The great toad demon Itokue has ruled the poisonous, dark and muddy Yomi for uncounted eternities, and that not even the oldest demons seem to recall her rise to power indicates that she is the oldest of all demon rulers - or the most powerful illusionist amongst them. Reguardless of which of these is true, Itokue rules her realm firmly entrenched, maneuvering the political landscape of hell with unparalleled acumen and remorseless, dispassionate brutality to enforce her law. All but the pettiest demons of Yomi are bound by an intriciate system of oaths of service, carefully constructed and magically enforced to allow for an optimal compromise between maximum loyalty and enough ingenuity to keep the government flexible. That being said, Yomi being paralyzed by contradictory orders from different princes and complex plots from other realms until Itokue herself intervenes it is not unheard of.
The ruling demons of Yomi resemble toads, frogs, and the occasional salamander or other amphibian, owing to the fact that Itokue's brood has taken over the plane long ago. Subservient to them are countless, most strange creatures modelled after different animals. Of particular note are the bakeneko, many-tailed demonic cats with the power to go unnoticed and animate zombies, who often slip into mortal realms to steal corpses for Itokues growing hordes of undead foot soldiers, and her fearsome elite of tsuguri-wielding sharkmen.
Gehenna, Courts of the Demon Lords
Smoke and ashes are the predominant geographical feature of Gehenna, the whole plane reminescent of long-deserted battlefields where opposing armies fought, long after the point where the conflict has become meaningless. This sense of pointlessness pervades the whole realm, and the political seperation and open conflict between the ruling demon lords keep the inhabitants in a constant state of war. Still, the rulers of Gehenna keep the conflicts down to a manageable level by a specific code of honor that would be considered outrageous in any mortal country, but nevertheless calls for some coordination and restraint whereever armies are fielded, so that the demons of Gehenna can still trade with other hells, collect souls, and plot against heaven as all demons do. Indeed, hailing from Gehenna gives one a certain degree of trust from other demons, who, not unwisely, reason that one is too preoccupied with infighting to be seriously involved in their conflicts. The code does call for a measure of honesty (or, more precisely, sticking at least to the letter of one's words) when dealing with outsiders.
Gehenna is home to warlike and treacherous demons, often covered in wicked barbs, having weapons for limbs, burning with eternal fire, or otherwise ever-ready for violent conflict. Optically, they tend to fit the classical imagery besides that - the typical native has a humanoid base with goat-related animal features, and red, white or black skin color.