“Its platinum loops shrink to a wedding ring, if you like.”
Metaphor
Compares the loops of the onion to a wedding ring that will ‘shrink’ which implies being trapped. The loops are ‘platinum‘ a very precious metal, which is ironic because the onion is not at all precious or desirable.
The implication here is that the further you get into a relationship the more serious it gets - ending in marriage.
‘if you like’ added to show that the relationship could go in this direction, but not necessarily.