CRAFTMEMORE2MM Hole 200PCS Tiny Grommets Eyelets Self Backing for Bead Cores, Clothes, Leather, Canvas (Silver)

How are the 2mm/3mm/4mm even used when the description says “back not included". Doesn’t missing the back render them useless?

Is this metal or plastic?

This size works with the crop a dile?

What makes them self baking? How do they stay secure?