hello samuel,
youtube videos are not good training material. Actually, except for one or two, the rest is just bullshit. That what fell off :-) is a positioning rubber - similar to the iPhone 6 and 6s, while the ring - it is a spacer ring to the glass level - allows you to even out by touching the surface with your finger - imperceptible difference.
spacer ring - do not lose it. Rubber - you can buy a new one with new glue. This will improve the comfort of pressing. Ultimately, only the glue can be replaced, similar to the buttons from 6 and 6s.
* regards,