Register an OLD device to my Apple ID

I have the same question as the OP, which clarifies later is has nothing to do with Apple Care.

I simply want to see all my historical (old but working) devices registered to my Apple ID. - all devices purchased new and from Apple, no second hand nor resellers.

A miriad ways to SEE which devices are already there. Yet, please anyone indicate how to ADD a missing one, e.g. an iPone 6, a MacBook 2010, etc.

Thanks in advance.