At the beginning of your upkeep, you may
castTo cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. A player may cast a spell if they have priority. See rule 601, “Casting Spells.”
To cast a card is to cast it as a spell. (CR 701.5) a permanent spell from your hand with mana value equal to 1 plus the greatest mana value among other permanents you control without paying its mana cost. If you don't,
scryTo “scry N” means to look at the top N cards of your library, then put any number of them on the bottom of your library in any order and the rest on top of your library in any order.
If a player is instructed to scry 0, no scry event occurs. Abilities that trigger whenever a player scries won’t trigger.
If multiple players scry at once, each of those players looks at the top cards of their library at the same time. Those players decide in APNAP order (see rule 101.4) where to put those cards, then those cards move at the same time.
An ability that triggers whenever a player scries triggers after the process described in rule 701.22a is complete, even if some or all of those actions were impossible. (CR 701.22) 1.