Can't answer your question as I'm also trying to understand but recently Graphene OS has been in the news.

Basically there are apps that won't work if they have not been authenticated by one of Google's APIs. Which means there are apps that won't work if it did not come from the play store.

After x tries, forces a pin similar to the biometric today

Basically pick a number and a spot on the wallpaper. When you unlock the screen shows a bunch of numbers and you move it till your number fits the spot. Takes a second.