It determines what enchantments to return when its resolution starts. If more enchantments go to the graveyard during the resolution, those additional enchantments are ignored.

You must return Auras if possible, even if this means enchanting an opponent's permanent with a good enchantment.

Auras can only be placed on permanents that were on the battlefield before this effect started to resolve. You can't put an enchantment onto the battlefield with Replenish and put an Aura that is also entering the battlefield onto one of those enchantments.