Zack is leaving Aerith's house to get to Biggs. He thinks of what Marlene said and feel a bit heartbroken here. Then, at the same time Cloud wakes up on wheel chair. The same spot we're ever shown in previous chapter that Cloud has vision or dream he saw her (in Zack's world) sleeping on bed. As they go out, it's the same sky with the rift. No Elmyra who has promised Zack that she won't let Aerith snuck out the house. Means, Sephiroth is gonna kill her and that's what Zack want to prevent. Aerith's date is like confirming what Zack is feeling if she liked Cloud. Their date is messed up, they aren't like couple. Cloud even doesn't know what Aerith means by "our place." It turns out that Sephiroth is actually lurking what Aerith is doing there as she hears his voice and see his black feather.
After meeting Biggs and escape from the reactor sector 6, Zack goes to the church, sitting on the stairs outside, Sephiroth suddenly show up, about entering the church, and throw Zack away. At the same time, Cloud & Aerith is having conversation inside but Zack dunno they're inside, he dunno Aerith's feeling for Cloud is romantically rejected because for him, she's a 'nakama' that she also question if her liking him is romantic or not. Sephiroth enters the church.
When Zack meet Cloud and fight with him in Edge of Creation, he doesnt know Aerith is killed by yelling "Save her!" to Cloud when they're separated. Zack returns to his dying world, in the church where previously we see Aerith was rejected by Cloud, and he feels her presence. Then that world is about to be destroyed by meteor. A kind of magic that symbolize Aerith's pink ribbon protect Zack here (I wonder where Zack lost the ribbon he tied on his hand when about to meet Biggs, though). His world then collapses.
Zack wake up in the church as if it's all just a dream. Which world does he exist now?
If it was the same world where he leaves Aerith's house, he'd return to check if she was still there. But... perhaps, this is the real Lifestream where he's supposed to rest. He chimed about the world unite again someday, perhaps meeting Cloud, led by the yellow flowers as the cinematic angle shoot it. However, the flowers is Aerith's favorite flowers (Elmyra talks to him), the church is her favorite place (where they're used to date), so it's symbolic that he also wish for reunite with her. If he met Aerith here, that means she wakes up, and he knows that means she has died. Though if he could, he would take back his life. And that means, he would take back her love (although he still dunno how Aerith's date with Cloud ends that symbolize she still love him).