I've given my homebrew's cosmology some thought, but it rarely comes up. I mostly play lower-level games and the planes are distant and mysterious to most of the characters.
There's an astral plane that exists "above" the material plane. Most spells that reach the astral plane must be cast in high places such as mountains, most astral creatures (such as angels and various other spiritual beings) manifest on or near mountains. The astral plane is often associated with the sky because of that and it's appearance. It's a vast, silvery void where the various portals appear as motes of light in the distance.
The Seven Heavens are one of the best-known planes in the astral because mortals seek it. Angels travel the astral constantly, both on patrols and to battle evil spirits spread throughout the universe. Only a few mortal souls are selected to journey to the Seven Heavens in life or in death. Although it's a magnificent place, it's a place of labor and purpose--mortal souls there have a good reason to be brought among the angels. There's also a peaceful, restful place for those mortal souls that weren't taken to the Seven Heavens, known as Elysium. The vast majority of good-aligned mortal spirits reside there and calmly await the chance to join the angels in the Seven Heavens. There are said to be other planes of existence scattered throughout the astral, but these tales are speculation to most mortals.
The very top level of the Seven Heavens is a magnificent walled garden inhabited by dozens of powerful angels associated with light and fire. They guard a gate to the place where the entire universe's Creator dwells, but mortals are not allowed to pass through there. Angels guard the gate with swords of fire and have repelled every attempt to attack the garden itself. There are no accounts of any mortal or immortal spirits ever passing through the gate. The major "good" religion of my homebrew holds a prophecy that the Creator will eventually open the gate and allow his followers to spend eternity with him, but that time has not yet come to pass. Even the angels don't know when that will be. This deity issued prophecies and edicts to mortals and then withdrew from the universe, the angels guard the gate and have patiently awaited the Creator's return for millenia.
There's an ethereal plane, where ghosts and various incorporeal spirits dwell. People usually describe it as "misty," but that's not always the case. The senses are strange here: sounds are slightly muted and things in the distance are a bit blurred and indistinct.
Below the ethereal plane is the Underworld. That's a nasty place full of ghosts, shadows, specters, and similar critters. It resembles a bleak, barren version of the material world under perpetual twilight. The only notable features are mountains and multiple rivers. Mortal souls that appear in this place do so at Mount Erebus, the "closest" place to the material plane. When people die, they seem to fall asleep and "wake up" in the caverns of Erebus. Most of the Underworld is dominated by a single huge kingdom of ghosts called Hades, ruled by the ghosts of three ancient kings. They dispatch ferrymen to collect the souls of the dead and escort them to Hades. That's because various devils, demons, and other evil spirits try to capture and exploit the souls of the dead. Angels make occasional forays into the Underworld, but maintain no established outpost there.
It's possible (although rare) for a strong mortal soul to elude capture at Erebus and find a way to escape by navigating the caverns and eluding the ferrymen. Such spirits find their way to the ethereal plane where they manifest as ghosts.
Below the Underworld is a place called Tartarus, where primeval creatures from previous epochs are imprisoned (most notably, the titans and several powerful demonic entities). Few living creatures have even heard of Tartarus, much less been there and come back from it. In game terms, it most closely resembles the shadow plane and "funnels" into the negative energy plane as you reach the nadir of the universe.
The Abyss and the Nine Hells seem to exist both somewhere in the Underworld and the Astral Plane--devils and demons roam both planes and gates to their realms are found in both. Cosmologists aren't satisfied with their locations and it's a subject of debate. Most of their major battles take place in the Astral Plane because it's vast and empty, with no territories to despoil and no mortal souls to imperil. In the ancient past, the devils actually won the Blood War and subjugated the Abyss. Although the devils have technically won and receive tribute, it's really more of a stalemate. Demons outnumber devils by about a four-to-one margin and they could absolutely win the conflict if they'd ever unite under a single banner. Demons are exploitable because they're chaotic. Devils won all their major engagements with the demons only because they are better organized and disciplined. Although the devils managed to decimate the first level of the Abyss and kill several powerful demon lords, these victories are far too costly in terms of resources and devils slain to give any real hope of "conquering" the Abyss.
There's an astral plane that exists "above" the material plane. Most spells that reach the astral plane must be cast in high places such as mountains, most astral creatures (such as angels and various other spiritual beings) manifest on or near mountains. The astral plane is often associated with the sky because of that and it's appearance. It's a vast, silvery void where the various portals appear as motes of light in the distance.
The Seven Heavens are one of the best-known planes in the astral because mortals seek it. Angels travel the astral constantly, both on patrols and to battle evil spirits spread throughout the universe. Only a few mortal souls are selected to journey to the Seven Heavens in life or in death. Although it's a magnificent place, it's a place of labor and purpose--mortal souls there have a good reason to be brought among the angels. There's also a peaceful, restful place for those mortal souls that weren't taken to the Seven Heavens, known as Elysium. The vast majority of good-aligned mortal spirits reside there and calmly await the chance to join the angels in the Seven Heavens. There are said to be other planes of existence scattered throughout the astral, but these tales are speculation to most mortals.
The very top level of the Seven Heavens is a magnificent walled garden inhabited by dozens of powerful angels associated with light and fire. They guard a gate to the place where the entire universe's Creator dwells, but mortals are not allowed to pass through there. Angels guard the gate with swords of fire and have repelled every attempt to attack the garden itself. There are no accounts of any mortal or immortal spirits ever passing through the gate. The major "good" religion of my homebrew holds a prophecy that the Creator will eventually open the gate and allow his followers to spend eternity with him, but that time has not yet come to pass. Even the angels don't know when that will be. This deity issued prophecies and edicts to mortals and then withdrew from the universe, the angels guard the gate and have patiently awaited the Creator's return for millenia.
There's an ethereal plane, where ghosts and various incorporeal spirits dwell. People usually describe it as "misty," but that's not always the case. The senses are strange here: sounds are slightly muted and things in the distance are a bit blurred and indistinct.
Below the ethereal plane is the Underworld. That's a nasty place full of ghosts, shadows, specters, and similar critters. It resembles a bleak, barren version of the material world under perpetual twilight. The only notable features are mountains and multiple rivers. Mortal souls that appear in this place do so at Mount Erebus, the "closest" place to the material plane. When people die, they seem to fall asleep and "wake up" in the caverns of Erebus. Most of the Underworld is dominated by a single huge kingdom of ghosts called Hades, ruled by the ghosts of three ancient kings. They dispatch ferrymen to collect the souls of the dead and escort them to Hades. That's because various devils, demons, and other evil spirits try to capture and exploit the souls of the dead. Angels make occasional forays into the Underworld, but maintain no established outpost there.
It's possible (although rare) for a strong mortal soul to elude capture at Erebus and find a way to escape by navigating the caverns and eluding the ferrymen. Such spirits find their way to the ethereal plane where they manifest as ghosts.
Below the Underworld is a place called Tartarus, where primeval creatures from previous epochs are imprisoned (most notably, the titans and several powerful demonic entities). Few living creatures have even heard of Tartarus, much less been there and come back from it. In game terms, it most closely resembles the shadow plane and "funnels" into the negative energy plane as you reach the nadir of the universe.
The Abyss and the Nine Hells seem to exist both somewhere in the Underworld and the Astral Plane--devils and demons roam both planes and gates to their realms are found in both. Cosmologists aren't satisfied with their locations and it's a subject of debate. Most of their major battles take place in the Astral Plane because it's vast and empty, with no territories to despoil and no mortal souls to imperil. In the ancient past, the devils actually won the Blood War and subjugated the Abyss. Although the devils have technically won and receive tribute, it's really more of a stalemate. Demons outnumber devils by about a four-to-one margin and they could absolutely win the conflict if they'd ever unite under a single banner. Demons are exploitable because they're chaotic. Devils won all their major engagements with the demons only because they are better organized and disciplined. Although the devils managed to decimate the first level of the Abyss and kill several powerful demon lords, these victories are far too costly in terms of resources and devils slain to give any real hope of "conquering" the Abyss.