Although not all demonic beings do inhabits in The Underworld, which also serves as the final resting place of deceased souls who has accomplished tremulously amount of evil who becomes tormented until their humanity is stripped away and they become Demons,
it's also a place where evil beings resident most notably demons and are with reasonable assumption safe from forces of Good where they can practice their black magic, many demonic species are born, hatched, or created here,
It is a place where even evil many demons have negative feedback due to which contains deep underground chambers, caverns, and volcanoes, and due to the limited light which only comes from lit torches, fires, and even cracks in the walls.
Although unlike their angelic "cousins", demons are a malevolent life-form as greater majority of demonic beings are unable to feel human emotions and not always inherently evil, although they are susceptible to it as they would without a doubt use any means necessary to accomplished their own dark goals even going so far to work with beings of the light.
Despite they are all demonic beings and many of them do share a common purpose of causing expressive of cruelty to the innocent(s) and good being(s) and to influence and infiltrate the mortal world, some of the evil forces rather prefer on vanquishing each other or power stealing do to mainly with the dramatic differences in their natures.
Fortunately, in face of great crisis when it's concerning both parties from good and evil, there have been shown to be able to put aside their differences and work together effectively, showing that both do value the other as with the creation of The Tribunal which is neutrally comprised council of two upper-level demons and two elders.