Big-Bang:

[source: https://www.reddit.com/r/HonkaiStarRail/comments/1426iym/comment/jn3e51h/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button]

In the very beginning, when nothing existed, the Lord of Myriad Worlds made a decision to create a universe where immortal gods would live. These gods embody various concepts, similar to the Aeons. Since the creation of the universe, these gods have been engaged in never-ending battles, to the point where they have even forgotten the reason why they are fighting in the first place. Observing the eternal suffering and wars occurring throughout the universe, the Lord of Myriad Worlds realized that the universe he created had flaws. Specifically, the immortality of the gods resulted in a lack of change and progress. To address this, the lord introduced a completely new concept called "death" into the universe.

Sometime later, the god who embodies the concept of death, Thanatos, following the order from the King of Gods, Zeus, goes on a killing rampage to eliminate all the other gods in the universe. Thanatos herself is physically weak, but as the embodiment of death, she can impose this concept upon every god she encounters, killing everything in her path. Zeus will later come to take the deceased gods' concepts and absorb them into himself.

As the war reaches its end, only Zeus and Thanatos remain as the sole gods in the universe. Zeus, having absorbed all the concepts of the other gods killed by Thanatos, decides to turn on her, so he can absorb her and become the sole ruler of the entire universe. However, Zeus cannot kill Thanatos because she is the concept of death and exists in a state which is neither life or death. Thanatos then grants the concept of death upon Zeus, causing his death. When Zeus dies, all the concepts of the other gods he absorbed explode out of his body, triggering a transformation and expansion of the existing universe, akin to a magical big bang that takes around ten quintillion years to complete.

This newly formed universe came to be known later as the Imaginary Tree (a term introduced by the scientists in the universe), where each leaf symbolizes the worlds existing within isolated Imaginary Spaces.

The Hoyoverse games that are confirmed or at least implied to be the leaves on the Imaginary Tree are GGZ, Honkai 3rd, and Genshin. Meanwhile, HSR takes place in multiple different leaves due to the Astral Express' ability to travel through the Imaginary Space. 

 Honkai-verse summary:

[source:https://www.reddit.com/r/honkaiimpact3/comments/16rbfwe/what_is_the_tree_of_existence/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button]

*The Hoyoverse or Honkai Universe or miHoYo's connected universe is a quantum multiverse. Why?

*In Honkai Gakuen Thanatos was created by the Lord of Myriad Realms as the Gakuen world's concept of death. She eventually kills Zeus who had become a singularity by collecting all of the other god concepts and upon death exploded. This is the Gakuen Universe's Big Bang event. (Compare that to the recent Primordial Sea creation lore of Genshin.)

*The lord came from outside of the Gakuen Universe which is the Sea of Quanta an abtract concept of true entropy bound only by quantum mechanics.

*Out of the Sea the Imaginary Tree spontaneously formed. It is an abstract concept of creation using quantum tunneling and vacuum fluctuations to make use of the energy potential of the Sea of Quanta.

*In Genshin which is less educated in modern scientific theory the tree and sea might be interpreted as the Ancient Greek Primordial Gods Ananke, Goddess of Inevitability and Chaos, God of the Void.

*In Star Rail the weapons are called Light Cones which are quantum mechanical concepts defining all possible positions of all beings in any universe.

*It is possible to travel backwards in time by manipulating Light Cones with intense gravity like that found in the densest star cores that collapse into black holes.

*Stellarons are star cores which cause the phenomenon Fragmentum a fracturing of reality in a localized area similar to this type of time travel.

*In this interpretation a person travels back in time but along their own World Lines which are all the possible Paths they have within the Light Cone.

*The Trailblazer possesses a Stellaron inside of them and has already been shown to adopt new Paths. This can also simultaneously be interpreted through a religious lens as the Trailblazer on the path towards attaining nirvana under Buddhist beliefs.

*From the outside, observers would not see the myriad of Paths taken by the Trailblazer because they collapse into a singular common sense path according to Richard Feynman.

*On the other hand Genshin follows Erwin Schrödinger where an observer selects for the common sense interpretation of reality across the many World Lines. In Ei's Second Character Quest she alone remembers a time without the Sacred Sakura tree because she exists on a World Line that goes back in time to plant the tree. Observers on the other hand collapse the different World Lines so that only the path where the Sacred Sakura always existed remains.

*Time travel was made possible in Genshin by the God of Time Istaroth but she might actually have the Light Element and therefore time travel remains consistent with the gravity manipulation of Light Cones found in Star Rail.

*In multiverse theory the World Lines could all exist in parallel. In Honkai Impact this is shown through the Captainverse where characters from the main story like Bronya Zaychik exists in alternate lives like Bronie and Magical Girl Bronya. Extended to Star Rail she also exists as Bronya Rand. These are separate World Lines for Bronya across the separate Universal World Lines of the games.

*These Honkai universes diverged at a more recent point compared to Genshin and Zenless.

*Quantum mechanics is a part of theoretical physics and therefore not observed in practical reality. As such it is subject to change and different interpretations and a fictionalized version of it will likewise be flawed and incomplete. But miHoYo seems to be keeping their Hoyoverse up to date with new discoveries like the importance of imaginary numbers in defining reality.

Power-scaling: (I am very doubtful about this part)

[source: https://www.reddit.com/r/PowerScaling/comments/18ei3dt/comment/kco52q7/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button]

The absolute top tiers scale from H1C (Zues, Thanatos, Lord of Myriad Worlds, Aeons) to L1B (Kiana(Finality), GGZ Kiana/Mei, GGZ WoH, Outer Gods, Sa). There's also Ryusuke Muruta (Low 2C) and Otto (H1C durability, 5A Attack Potency). These are all based on multiple statements putting them at 11-12Dimension, usually based around the Imaginary Tree (Otto fusing with it, Outer Gods being able to recreate and destroy an infinite number), which is 11D. WoH, Sa, Mei, and Kiana(s) are based on Honkai and the CoF being stated to transcend all dimensions. WoH (and therefore Kiana/Mei) should scale massively above that (since the CoF is a fragment of the WoH). Kiana should scale to the CoF after fusing with it, and Sa was stated to be 1% of Kiana's strength.

High tiers scale from 4A (Kalpas, SAKURA, Final Fight Trio, Elysia, Kevin) to 3C (Lord Ravagers from HSR). The bottom half is based on those characters being able to one shot Durandel (based on SAKURA one shotting AHR Mobius, who's relative to HoT Mei, who's relative to DA Durandel), with the top half being based on Lord Revengers being stated to be able to incinerate galaxies.

Mid tiers scale from 5B (Sustainer and Full Authority Neuvullette) to 5A (most relevant HI3 characters, like Durandel, Mei and Bronya before Kiana became HoFi, 6Core Sirin, etc). The bottom half is based on the Narwhal being stated to be able to survive the planet's destruction. The top half is based on numerous statements (including direct interviews with the writers) putting Durandel at solar system level (meaning most of her attacks would probably be large planetary).

Low tiers are anyone who scales below the Silver Bullet, so 10C to 6 something (the entire playable cast of HSR, GI, the early cast of HI3, and most characters from the HI3 manga.