The concept of The Three Realms (三界, Sānjiè) is inherent in mortal. They believe that Immortals, and Gods live in heaven. Mortal Realm is where ordinary mortal life, while the underworld is where they will be judged, purified and finally reborn.
But the truth in the eyes of "Cultivation", The Three Realm itself is at the Small World (at same world). So-called Heaven is place where Spiritual Qi is abundant, and place where mortal can began their journey to cultivate Immortality. The abundant Qi also make the landscape become more beautiful and breathtaking as if they are in "Heaven".
Beside all of cultivator having a good- looking face and nice body because their bodies are free or contain only a few impurities, and nourished by Qi. This is also the reason why mortal often call cultivator with "Fairy" or "Immortal" despite they are not yet become Immortal.
The distribution of Qi is not not equal. There are areas where Qi is abundant and there are areas where Qi is scarce. There are even some who do not get any part at all.
Mortal realm identic with place or area where Qi is sparse or even there didn't have any Qi at all. Because of this, it was rare for mortal who born in this realm to having spiritual roots. This place also usually very far from the "cultivation" realm. Cultivator will feel uncomfortable or even vulnerable if they are in the mortal realm because of the scarcity of Qi.
On the opposite, mortal will feeling very comfortable when they are be in a place full of Qi.
This is also the another reason why mortal in mortal realm rarely saw a cultivator, some even never see or hear about cultivator in their entire life. All of the reason above is source of the misconception and ignorance, making these mortal mistaking cultivators as Immortal or God.
Of course in the "Heaven" or we will call it as "Cultivation Realm" in later to make easier to understand, mortal also live in that area. Usually mortal who live in the cultivation realm, their lifespan is longer than mortal in other place. Their face and body also much fair, and stronger.
Continuous exposure to Qi can make body feel lighter, nourish the cell, making the body become more stronger, healthy and they will have longer lifespan and fair skin.
Mortal realm identic with place or area where Qi is sparse or even there didn't have any Qi at all. Because of this, it was rare for mortal who born in this realm to having spiritual roots. This place also usually very far from the "cultivation" realm.
Cultivator will feel uncomfortable or even vulnerable if they are in the mortal realm because of the scarcity of Qi. But even so, the sects and clans still accept mortal at this area as disciple.
Sects or clans will send a team consisting of disciples at late of Qi Refining or Foundation Building will be lead by a Golden Cultivator to accept disciples from mortal realm once every few years, to become new blood for the sect and clans.
Prospective disciples are usually limited to between the ages of 6 until 20 years old that will be tested their spiritual roots. The bottom line of spiritual roots to be accepted by each sects or clan is also differ from each other.
Mortal realm can become exiled place where the sinner wil be thrown after their cultivation base being destroyed, and their memory becoming cultivator and anything about cultivation being erased or sealed.
It also can become the place where side branch of the clan will live. They usually divided into two group, branch that live in sparse Qi, and the other is on the "null" of
Qi.
28 Heavens (二十八天, Èrshíbā Tiān)
28 Heavens is from 7 basic realm multiplied by the 3 stages (early, middle, and late) is 27. The another 1 is for the Perfect stages or Half-step into.
So it's basically the stages of layer of cultivation realm, that cultivator will have in their journey in this small world.
The Eight Heavens Beyond the Three Realms (八重天, Bā Chóng Tiān)
"Beyond the confines of the Three Realms lie the Eight Heavens, realms of true transcendence where beings are free from calamity and reincarnation."
From that sentence, there's a few world that we can analyze.
"Transcendence" can also translated as "Ascended", it obviously have meaning these beings is not at this small world anymore, but they already ascend to the higher world.
"Free from calamity and reincarnation." Because these beings is not on the same world as human in the small world anymore, obviously they are free from "The Catastrophe and Rebirth" that happen to the Small Worlds.
Notes: Information about "Structure of Heaven, Earth, and the Three Realms" I get it from the novel'莫挨,我转修无情道了'or "Don't worry, I've turned to the Ruthless Dao"