Mt. Sumeru or Three Heavens Great Thousand Worlds (須彌三天大千世界) consists of many Heavenly Domains. For the vast majority of Heavenly Domains, there exists five dimensions and three planes of reality that make up the world. This is how the structure of Heavenly Domains within the story operate.
Overview of Heavenly Domains
Five Dimensions of a Heavenly Domain
Astral Realm
The Astral Realm is the universe as we know it, the material realm. It continuously expands at the speed of light and countless living beings are born within it. The Astral Realm is the core upon which the four other dimensions are centered upon.
Interdimensional Void
The Interdimensional Void is the dimension upon which those who cultivate attraction force to the extreme — cultivators — pass through to reach the Middle Realms. The Middle Realms, as well as all other Decaying Corpse Realms (the corpses of True Immortals whose bodies are akin to worlds themselves) are located in the Interdimensional Void due to their attraction force. This is the domain of the Heavenly Venerable of the Void. The Interdimensional Void is the heart essence of the Future King.
Underworld
The Underworld, also called the Netherworld — the domain of the Heavenly Venerable of the Underworld — is the dimension reached by living beings who fail to cultivate sufficient attraction force. Essentially, it is place where the dead goes. Nonliving souls cannot enter the Underworld; only living souls may enter when upon death. Ghost Path cultivators may develop a sense of the outskirts of the Netherworld, dubbed fittingly as Netherworld Perception. Souls that go to the Underworld, depending on their actions in life, will go to either paradise or hell. Regardless of which one, these souls will eventually reincarnate.
Source River
The Source River, also called the river of time, or the Fate Realm (運界), is the domain of the Heavenly Venerable of Time. This dimension regulates the passage of time in the world as well as the stars (stars, planets, and other cosmic bodies) of the Astral Realm. The souls of stars, the nonliving, and souls that do not fully form reach the Source River; it is not a place for living souls.
Eastern Heaven Flower Field
The Eastern Heaven Flower Field, also called the Flower Realm, returns the nonliving from the Source River and the souls from the Underworld back to the Astral Realm. It is the domain of the Sal Tree Heavenly Venerable.
Three Planes of Reality
The world exists in three planes of reality: Plane of Qi, Plane of Soul, and Plane of Fate. They are not distinct planes of existence, rather they are all connected. Throughout a cultivator's journey, they will explore all the three planes, no matter their cultivation method. Nascent Soul is where one begins to sense the Plane of Soul, and Sacred Vessel is where one cultivates the attraction forces of Fate.
True Immortal Realm
The True Immortal Realm is not structured like the Middle Realms when one ascends. Instead, it 'overlaps' upon the Heavenly Domain. It is a space where the planes of Qi, Soul, and Fate overlap. It's not a plane like the overlapping dimensions of the Astral Realm. It is a world that has always existed from the start, but could not be perceived by beings of lower ranks.
Inner Sea
The 'Inner Sea' refers to the dimension between Heavenly Domains. It is very arduous and time-consuming to cross, as Golden Shaking Bird says that for the three Vestige Liberation Immortals there it would take 70,000 years to reach the next Heavenly Domain.
Cycle of a Heavenly Domain
When a Heavenly Domain starts, it expands from a single point in space like the Big Bang. When a Heavenly Domain ends, it contracts upon itself into an infinitesimally small point, like the Big Crunch.
Structure of Mount Sumeru
Mount Sumeru is in the shape of an inverted cone. It consists of nine Heavenly Domains, with the Sun and Moon Heavenly Domain at the vertex, with the rest of the heavenly domains filling the rest heading towards the base. Sitting atop the base of Sumeru are 6 Supreme Deities watching over Mount Sumeru and it's cultivators.