Legends: Z-A centers its evolution system around Mega Evolution. 43 new Mega forms debut, and Mega Evolution can be triggered from the overworld, not just in battle. The game uses Legends: Arceus's action RPG format in Kalos, and trade evolutions use items instead of trading, matching the Arceus approach.
Your trainer needs the Key Stone (unlocked after main mission 9) and the Pokemon must hold its matching Mega Stone. In battle, a Mega gauge fills up over time. Once full, press R-stick to Mega Evolve. The Pokemon stays Mega for the rest of that battle encounter. Unlike X and Y, Z-A lets you Mega Evolve from the overworld too, not just in trainer battles.
Most Mega Stones drop from defeating Rogue Mega Pokemon scattered around the map. These are wild encounters with oversized, powered-up Mega Pokemon that attack on sight. Beat one, get its stone. A few stones come from ranked online battles through the Z-A Battle Club instead, like the stones for the Kalos starters Chesnaught, Delphox, and Greninja.
Yes. Z-A follows the same approach as Legends Arceus: trade evolutions are replaced with usable items. The Linking Cord handles Gengar, Machamp, and the others that normally need a trade partner. No friend required.
Both. Standard evolution works the same way it does in Legends Arceus. You choose when to evolve from the menu (no forced evolution on level-up). The Mega system sits on top of that as an extra battle transformation for fully evolved Pokemon that have Mega forms. Megas aren't permanent evolutions; they revert after the battle.
Z-A added dozens of new Mega forms that didn't exist in X and Y or ORAS. Some picks like Mega Dragonite and Mega Greninja were fan requests for years. The Mega Dimension DLC expanded the roster even further. Between the base game and DLC, the total Mega Evolution count is well over 70 Pokemon.
Very similar. You still evolve manually from the satchel menu, so there's no surprise mid-battle evolution. Items replace trade requirements. The big addition is Mega Evolution layered on top. Move mastery and Agile/Strong styles return from Arceus too, though they don't directly trigger any evolutions in Z-A the way some Hisuian evolutions required specific style usage.