How exactly does unicorn magic function? Is it held in a reserviour in your body that depletes and then refills itself, or is it another form of physical stamina, etc.?
Just like any action, using magic requires energy, which comes from your body’s energy stores. Even just the thinking and concentrating involved of course requires energy.
However, it’s not just a matter of energy in equaling energy out. The work required to move the moon would take far more energy than your body could generate (equivalent to septillions of sandwiches worth).
Part of the energy, beyond that required by your brain to think and concentrate, does come from your body, but you also tap into the ambient magic around you, what the ancient unicorns called Textus Arcanum, the Hidden Fabric. In a sense your own power acts as a catalyst, though not only as a catalyst.
The more power you have, the more of the ambient magic you can tap into, so your spell casting ability is limited by your own energy level, and it’s important to get plenty of food and rest. Habitual magic users do end up eating a lot, particularly alicorns who often use far more demanding spells.
Some think it’s funny that Celestia eats so much cake, or that I eat so many hayburgers, and while it might seem excessive to the average pony, really it is necessary. Not cake and hayburgers specifically of course, but energy rich food.