You gotta breed a silver chocobo to fight Sapphire Weapon under the Midgar Swamp. Then you have to fight Cheese Weapon on the moon that drops the rare item cheese. This lets you talk to Choco Billy for him to give you a side quest to breed cows for him. After you breed 2 Wonderful Cows and breed them, you get a gold cow. If you take the gold cow and ride it into Midgar, then go to the church where Aeris appears, Zack will be there and he will kill Cloud and take the Buster sword back from him and join your party.
But how do you even get to the moon? Ride some kind of magic fucking space whale or something? Don't make me laugh.
When you beat Sapphire Weapon, he tells the party about Cheese Weapon being on the moon. Cid then says "let's take the rocket to the moon." He then repairs it and you take the rocket in Rocket Town there. ---