Spoilers ahead.
Onto the magic system again. Magic in this shared reality works quite simply: exchange something for something else.
In the beginning of the universe, when the laws of physics that govern this world were made, "First" and with "Second" adding to it had made an additional law, the (WEL) Will Exchange Law, dictating that anything that has a form of will can exchange something for effects, meaning that one could exchange a gram of iron into energy, and with E = mc² this is 9×10¹³ joules, which is similar to the bomb dropped on Nagasaki.
This couldn't be allowed, so Third (Main Body Lehatot/The Creator) created the CM (Causality Membrane) that spread across the shared reality and fused with it.
CM makes sure that spells cannot convert most mass to energy directly, and that only specific levels of wills can access the WEL, meaning that the only things possible to exchange are forms of energy or mass made to store energy like Mana. Note that Spirituality is not a form of energy as it is an assigned value of a creature by the CM; this can make someone into a spirit if they die without exhausting their Spirituality.
Such masses include and are not limited to Mana Cells, Fat, Unstable Elements like Plutonium, etc.
Mana Cells are evolved by the creatures of this shared reality so as to store exchangeable energy safely; they vary by birth and they cannot increase or decrease natural compression by training, but mana itself can be increased or decreased if used/unused periodically, though limited by the compression factor.
High rank users get more mana with training but decrease more drastically if unused as well.
But they can store more energy temporarily if it's more compressed; if higher compression is given to a lower compression capacity, the energy will eventually be decompressed naturally; if lower compression is given to a higher compression capacity, it will naturally be compressed.
"First" is the first Creator of the world, who abandoned the world soon after; he is no longer influential inside the Shared Reality and is confirmed to have had a male identity. His name was Erased and actions were assimilated by Third, making It the first Creator while still bound by its original sets of rules.
"Second" is the Second creator; it has domains over the moons and lunar cycles. His name was Erased and actions were assimilated by Third, making It the Second Creator while still bound by its original sets of rules.
"Third" is Lehatot (CM/MainBody); it has domains over many things in the shared reality with a wide variety of different avatars, one for each world, not just limited to AHIH who has domain over Sound and Music; it is the one that made Instability accessible to the residents of the world. It has limited control over the period of a few months before the (TYZ) True Year Zero of the World, but it can control the definitions of the actions and items that are undefined in that period.
It used this to insert itself into every bit of the world as its True Creator by using The Mother Of Salvation to use Instability to bring It to the world in the Pre-Timeline, causing Time to be transmigrated into the body of her daughter who died poisoned due to a political plot in a Count Estate.
The poison contained the will of AHIH, planting the dormant identity who was held by CM inside the body who awakened after Time left the timeline along with the dormant Identity of Salvation who still survived due to the Spirituality tied to her body; this being the TYZ.
This made "Third"'s ability to control the shared reality be above First and Second, allowing it to bypass restrictions, but it formed a scar in continuity due to the fact that The Mother needed manifested Instability to bring "Third" inside, but "Third" was the one that manifested it in the first place.
Also note that Time's transmigration was originally the action of "First" but his actions were assimilated by the "Third".
CM also makes sure that events happen so as to bring in TYZ (The awakening of the avatar) and makes sure that Causal Points happen in some shape or form as to reach the proper End of the world and not risk Extermination of the Timeline by the Underworld.
Later I will explain how exactly WEL exchanges energy for effects.