Modern Horizons 2 Previews

[FourDogsinaHorseSuit]

Also, how do the rules work there? Stage becomes a Saga, no? And Sagas gain counters each turn, no? It sounds cool, but I’m confused.

Short answer is layers. Those chapter abilties resolve to create layer 6 effects which are applied after stage's normal layer 1 effect.

Long answer:
Stage is now a Saga. It has 3 Chapter Abilities and 2T: Copy something else.
On your First Main it's chapter abilities triggers and now it has a layer 6 Ability adding effect, T: Add C.
On your next first main, chapter two triggers and now it has a new layer 6 ability adding effect, 2T: Make a Karnstruct.
Now if you copy comething else, the copy will be applied in layer 1, Copy effects. So Your stage has copied a Basic Mountain. It copies everything that is a mountain. Basic Land -- Mountain, T: Add R. But then in layer six, those two other abilities say "Also, this permanant has an effect which added abilities to it: T: Add C and 2T: Make a Karnstruct.