I don't understand the new zombie system

I downloaded the demo to get a taste of the game before it officially releases. I like a lot of the changes they made; but the way zombies work now seems odd. They need perks/mastery to perform tasks? Yeah, that's fair. They need red skulls to have access to perks? Ok, fair enough. But how do they actually GET perks? I couldn't figure it out. Can someone please explain? The tutorial wasn't very clear, and I don't want to be stuck on this when the game drops.