Aight I'm bored so here's some info on how mana works.
"Mana" in this world is basically just a type of energy. It can be interpreted as qi, chi, chakra or mana. Cultivation techniques will differ on how it is interpreted. Not the actual wuxia cultivation stuff, just techniques to grow in power. I'll be referring to it as mana though since this is the common use of it.
For the current continent Cain is in, it'd be techniques to increase magic circles. With an increase in magic circle count, comes an overall increase in physical and mental capabilities; the amount depends on the quality of the technique.
Mana is controlled through language. In ancient times it'd be the ancient language imbued with mana and carrying intent or will. Some other methods are through magic circles, chants, and visionary magic. Mana imbued words do nothing if they have no will or intent attached, they're just fancy bois.
Chants are obvious, just speak some funky stuff and magic happens (pun always intended). Magic circles and visionary magic carry a type of language; the language of the mind, a.k.a. vision or imagery.
Magic circles are kind of a roundabout way of it though. Peeps study it from a textbook, relate the magic circle to a spell, and now it becomes the thing they imagine when they try to cast it. Visionary magic is the closest to the original form of manipulating mana, but it depends on the imagination of the spell, which can fluctuate a lot depending on the person, thus is also the most fragile.
The original way is to use one's will, the language of the soul. Every method is derived from this form. To become a god you cultivate your soul to the extreme for precisely this reason. The limit to how much one can control mana depends on how strong their soul is.
Most cultivation techniques don't cultivate the soul, but rather strengthen it like a muscle, continuously injuring it by slowly controlling more mana than possible then letting it heal. For the magic circle techniques, most of them will direct the user to store extra mana in the area surrounding their hearts and continuously control it so it stays in that area, thus tempering the soul.
Almost all of this is unintentional though, the techniques have them gather the mana at the heart so when the time comes they just need to form the circle and get promoted. Keeping the mana in the area around the heart is like vaguely controlling it, while forcing it into a new circle is controlling every particle. This creates the bottleneck of promotion.
Cain controls mana in the original way (there's a conspiracy behind this) but he thinks he's using Visionary magic. There's a root form of magic circles, from which the current continents magic circles are derived from. Cain's magic circles follow the former at first, but slowly shift to the latter since it's the form he's exposed to the most. The switch occurs in the beginning of Day 4, when he sees Victoria cast a small flame.
There's not much difference between the forms of magic circles, the root form is harder to understand. Thus, the founder of Ouroboros' magic system (the current continent) created a new boi that was easier to remember and understand, but still wasn't decipherable to the point just about any genius could pick it apart and understand how it was created. The power difference and efficiency is negligible until it gets to hundreds of millions of mana per magic circle.
The magic circles are encoded with a cypher from Earth and then use Greek letters. The chant isn't encoded in the cypher so it's not just incoherent sounds and there's some logic. Ping me in discord if you want the full story behind the creation of the magic circles of Ouroboros.
Keep in mind... there's pretty much a conspiracy behind everything at this point and Cain's theories aren't always right. Don't forget to question everything.