Well, magic is usually “Mana” or some equivalent, in many stories you have to build up a mana reserve to use magic, it look like the body produce it or absorb it naturally, i’d lean on the option that mana is absorbed because in many stories the characters are way faster to fill up the mana reserves than fill up the energy reserves and usually they also explain that you can find mana everywhere and in the air so my guess is that we absorb it using our lungs or skin contact

So they have ““infinite energy”” (which is not infinite but it’s so big that we don’t care about it finishing soon, like the heat in the Earth’s crust) Probably there is also a natural process that release new mana like the decay of organic matter or some plant or bacteria that synthesise mana