Most of this stuff has already been explained in the past.
Everyone, even magic-less people, have magic energy dormant in their body. This isn’t stored in an organ or anything, its just an invisible essence, and each person has a limit to how much they can hold, which also increases the more they use magic and the more they age. If you use all the magic energy in your body, you aren’t affected at all, you just can’t use magic until you recharge. People would naturally recharge energy by sleeping, eating, etc., but since this is a game, its limited to only charging.
Charging is your body taking in magic energy from the world nearby
Pain inflicted from magic is your own body’s magic energy conflicting with outside magic energy, since magic energy when used in attacks behaves differently to the “natural” magic energy in the air, sea, and nature. This is why magics like water and wind are harmful.