Before a match you build an 8x8 main book plus a 16-spell sideboard - singleton, owned, and locked. During the duel every spell in that book is always castable if you can pay for it. Casting costs mana and casting minutes; bigger spells cost more of both.
Because nothing is hidden behind a random draw, ArcoMage rewards planning: sequencing your spells, baiting counters, and holding the right answer for the right moment. The only luck in the duel is your opponent's. See Circles and Casting Time for how spell power trades against the clock.