To put it in terms humans could understand, the Broken Domain could once be seen as a complete hollow sphere. It enclosed the Inner Planes while being enclosed by the Outer Planes, layer upon layer surrounding the Material World—the Main Dimension—like an onion.
However, when the ancient gods left the Main Dimension and Earth to seek other places to settle, their circumstances varied.
The most powerful ancient gods created and occupied the inner layers closest to the Main Dimension, building realms of their own there.
For example, Set's Sea of Snakes; Chthon's Shining Realm; Avalon, created jointly by Oshtur and her good sister Gaea but inhabited by neither; and Yggdrasil, the last to leave Earth, which transformed its own body into the "Nine Realms," a home for living beings and friends.
The Inner Planes were relatively closely connected to the Main Dimension. Back then, they were ideal places from which one could freely enter and leave Earth. They could even be regarded as locations within the universe of reality, reachable through simple physical means, such as spacecraft or Cosmic Flight.
As for the Second Generation Gods later carving out a piece of the Inner Planes to live in, calling it the Realm of Gods—such as the Greek Pantheon, the Hindu pantheon, and many ancient gods who had simply given up on themselves—that was all much, much later.
They were able to claim territory there not because they were especially powerful, but purely because the four great ancient gods were not home, and Yggdrasil could not be bothered to deal with them.
Still, the Realm of Gods was ultimately destroyed by Gorr the God Butcher. Remember the massage chair Su Ming picked up? That was where it came from.
Back then, the ancient gods who were a tier weaker had no