Zack, meanwhile, can just hope from world to world because he is pure spirit and doesn't know his physical body is dead.
I was chatting about this yesterday or the day before, and I think that while at the beginning he wasn't aware that he was dead, by the end he knew it.
Zack's scenes, while few and in-between, really do hold a lot of importance and I believe they serve both as the Lifestream exploration and understanding of the Lifestream (coupled with Aerith's dream world and Sephiroth explaining it all right after to Cloud) and as character development. It also serves, in the grand scheme of things, to explain that one line "
Her first love is at all times by her side" (about Aerith), which does clashes with CoLW!Aerith who is very Cloud-centric still.
First, for the question in itself, Zack is for the first time confronted to a situation where just being his usual cheerful self isn't enough: he has to think and we do see him perplexed by a few things:
- why is Bigg's description of Cloud so different from the Cloud he knows?
- how could Aerith meet and fall in love with Cloud when he was with him this whole time?
- why can't he save Aerith but Cloud can?
- why is everyone OK with the idea of the world ending and dying? (I think it will be a stark contrast with when Meteor will be summoned by Sephiroth)
All these have answers tied to him being dead and being in a place that knows past-present-future as one thing.
Biggs, who is a thinker, doesn't serve only as foil to him, but is there to stop him and make him think. They try to elaborate hypothesis, why them, etc. To us, the players, it becomes clear that Biggs accepted his death as we don't see him come back once he attacks the Shinra troops - Zack is not there yet, and furthermore, even if he accepted his death, the Planet has more in store for him.
But to be back to Zack's POV, he also has a moment in the Lifestream where he follows the yellow flowers (just like Aerith does), and when he meets Cloud, they clearly aren't in the same plan because he has to touch him to break the veil, so to speak. Afterwards, he spends his time crashing from church to church (IIRC they are more and more derelicted aren't they?), as CoLW!Aerith dies with all her worlds. And, a last one is built, as "our" Aerith dies in the real world: this is the one where Zack arrives.
It's pretty clear that with him trying to understand, at the end when he finally gets into Aerith's new dream world (her promised land, once again I am 100% sure about that), he understands that he is powerless, that he's not "alive" anymore, and all he can do is wait (yes, I believe Aerith will come back to this place and they will meet again, because while CoLW!Aerith's promised land was about meeting Cloud, "our" Aerith's promised land will be about being with Zack in their cherished place).