>>42437763Megas and Gigantamaxes are fused into the same mechanic: Burst Evolution. Burst Evolution can happen once per battle and lasts for a maximum of three turns. A held item is not required to do this. During that time, their Base Stats increase by a total of 100 and their abilities or typing may also change. In addition to normal moves, a Pokemon in a Burst Evolution can unleash a Burst Move at any time. Bust moves combine Z-moves and G-Max moves by being powerful attacks that may also have other effects. However, immediately after using a Bust Move, the Pokemon that used it will immediately revert out of its Burst Evolution, regardless of the number of turns that have elapsed. All Pokemon that currently have either a Mega or Gigantamax will have a "default" Burst Evolution. For example, the Burst Evolution for Charizard is Mega Charizard Y. Mega Charizard X and Gigantamax Charizard are available as Burst Evolutions if Charizard is holding special items. Design-wise, most Gigantamax forms will be tweaked to put them on the scale of Mega Evolutions.
For Pokemon without a Mega or Gigantamax, they can gain a Burst Aura instead. This encompasses the Pokemon in a bright Aura and increases all of their Base Stats by 10, except for their highest Base Stat, which increases by another 10 points. Pokemon in a Burst Aura will not change abilities or typings. Burst Auras last 3 turns. A Pokemon in a Burst Aura state can unleash Burst Moves, but they too will immediately revert back to normal after the Burst Move is executed.
You can have either ONE Burst Evolution or ONE Burst Aura in a single battle. You cannot have both.