How to set up duplicate YubiKeys with OpenPGP in 2021: computer security could be so much better right now

[With Addendum to chapter 8 regarding deleting all secret keys on the computer to improve security even further by confining secret keys to the YubiKey when using Kleopatra on the desktop]  The fact that this blog entry is so long (or even necessary) is clear evidence of the abject failure of the computer industry to…