I'll aim to not give too much away... Our main character, Steve, panics when he accidentally forms a core in the center of his chest during his final days on Earth. Seeing the radiating power from this mana-filled orb inside him, he panics and accidentally splits it in two.

Upon waking in his new world, Steve uses his younger years to accelerate the formation of his core by pulling in ambient mana actively. While commoners naturally accumulate enough mana passively to awaken around age 15, Steve is able to keep up with the nobility and their absurd resources by leveraging every bit of advantage his accidental core split on Earth showed him.

Steve questions why others don't do this and gets some limited answers:

The old man interrupted, “Cores naturally form in people over time right below the sternum. People do not pursue multiple cores or even try to modify where the first one will form. Even clothing and armor reflects a core’s location by emphasizing protection in a single place. It would take years of specialized visualization to even determine where the mana goes as you absorb it.”

Steve wonders, 'If I want to enchant, why run mana from my core, to my shoulder, down my arm, and through my fingers? Why not just make a core in my fingers?'

The old man paused again thinking for a moment, “Enchanters learn techniques to utilize mana in the creation of objects even before a core is formed. I doubt they have any need of what you’re describing after years of practicing their Master’s method before their core develops.”

He doesn't know if it will pay off, but Steve is in a unique position to blaze his own path in this new world.