I’m still a noob, yes. I wondered if you can have two or more faces, vertices, or edges mirror each other so that when you edit one, the other edits itself inversely. Please note: I’m not asking about how to create symmetrical models using the mirror modifier. I know a lot of noobs have asked about that. Thank you for your help!
You can select any amount of verts, edges, or faces, Shift-D to copy them and the Ctrl-M (and an axis X,Y,Z) to mirror that copy. But, This will not actually link the verts into a mirror relationship. Outside of the Mirror modifier and the older Xmirror option, I don’t know of a way to set a mirror relationship for selected verts.
BTW: Welcome to BA forums!