They seem unnecessarily opaque about it all.

I just made one for PayPal using my MacBook which seems to have ended up in Bitwarden rather than the mac thing. But there's nothing in Bitwarden to say list all passkeys. Not sure how I check elsewhere. Maybe they should email you "you have created a paypal passkey in Tim's Bitwarden" or something. Then at least you could search the email for "passkey"?

I wonder if I can use Bitwarden on another device with that? I honestly don't know.

You can't search for passkeys, but you can search for logins by URL and check the much shorter list for passkeys. The good news is that having the passkey in Bitwarden means you can use it on another device.