David wrote:

Can I confirm that they are like pop rivets (that is, they fit through a
hole then are pulled back to deform a tube) except that you can bolt and
unbolt things once they are set?

Pulling back done either by the bolt you plan to use, or a stronger bolt/
stud/whatever?

Again, a bit like wall plugs for fixing through plasterboard?

Yes, you fix them blind into metalwork and they give you a threaded hole