It's a digital key.
Unlike physical objects they may reside in a TPM, a software vault, an export/backup, or any combination thereof. You may or may not be able to recover or migrate them, depending on where/how they were made.
Therefore you may need multiple per service, or maybe not. Services which only allow one may end up locking you out with no recourse. You get to find out.
None of this is obvious or self explanatory to normies.
Probably because these caveats and weird behaviours are platform-dependent, not really the passkey’s fault.
Passkeys really are not any more difficult to explain than 2-factor authentication. Anyone who’s currently been able to actually create an Apple or Google account and successfully navigate their devices up to a passkey screen will be able to grok how it works.
People around here really ought to stop thinking users are complete idiots. Hell, you don’t even to scroll that far to read people calling users “normies” for crying out loud. What is this? High school?