not connected at all. glueing screen and glass together is ONLY marketing issue, there's no other reason behind this.
I've worked at a phone repair shop, and believe me, they were a pain to get them clean! Once opened, it never is as good as before, while the screens glued to the digitizers had never problems. This is why they are not separate anymore.
My Nexus 5 does this; as I discovered when I attempted to purchase a replacement screen, but inadvertently got only a digitizer.