Z Mega Stones are not in Champions yet, so this answer will only cover ZA for now.
In Legends ZA, all Mega Pokemon use all their moves as Plus Moves, with the Mega Z Pokemon (Absol, Lucario and Garchomp), being no exception. However, they also uniquely have all their moves' charging time set to 0 frames. A "frame" is (as far as I could find) 1/30th of a second. For context, most physical attacks have charging times of approx. 10 frames, ranged attacks have a charging time of 20 frames, and Geomancy (the longest in the game) has a charging time of 80 frames.
Best source I could find
As you can imagine, this makes them amazing users of powerful moves that have been nerfed in ZA by having a large charge time, with Mega Garchomp Z in particular using moves like Dragon Rush, Earthquake and Draco Meteor to often wipe entire maps in the Ranked battle mode.
Their only nerf, and second difference to normal Megas, is that they burn through your Mega Energy approximately twice as fast as non-Z Megas, so they will often, for want of a better phrase, Mega Devolve in drawn out battles.
