This transformation can only happen once per battle, and will last until the battle ends. To activate it, players need a Tera Orb to Terastallize a Pokémon, which will need to be recharged after each use. Players can charge their Tera Orb by touching crystals overflowing with Terastal energy or by going to a Pokémon Center.
As for more bonus items, those who will purchase the game digitally via the Nintendo eShop will get an Adventure set via a serial code and it is available up until February 28, 2023. This set comes with the following items: Potion x10, Full Heal x5, Revive x3, Ether x3, Rare Candy x1, and Nugget x1. To those who will buy the double pack, they will get two codes (one per game) that will reward them 100 Poké Balls.
[Additional details via the official Pokémon press release]