Trust me, the premise is very dumb.
In the first episode, the main character is like "hey how'd i become a zombie?" and the dude who revived her says, verbatim, "a little of this, a little of that, and boom, you're out of the grave", and the topic is never touched upon again.
The girls are also death metal idols, because their zombie noises sound like death metal screaming, and the fact that they're dead means they can headbang with the best of them because they can break their necks with ease.
Death metal also "awakens" their old personalities, and turns them from mindless zombies into normal girls who just happen to be dead.
I don't understand it either.
I still don't get why they decided to split the FFVII remake. Sure, it's gonna be a big game, but FFXV managed to fit on one disc, why can't this game?