The on-screen navigation buttons like this:
http://img.wonderhowto.com/img/71/8...pth-visual-look-android-5-0-lollipop.w654.jpg
See how there's the triangle, circle and square at the bottom? They function as back, home and multitask buttons. That's Google's direction for the navigation instead of the physical and capacitive buttons that the S6 has.
And OP, there's no way to activate on-screen buttons from the stock S6 software. If you want to do that, you may have to look into custom roms.
Sent from my SM-G920I