Many places let you enter and name multiple passkeys. So you as your keychain one and name it "keychain". And also add phone and call it "whatever phone" then use either.
Personally I only use devices that don't sync and can't be copied for security reasons.
So my problem is that I keep forgetting which device, browser or app I used when I created a particular passkey. I'm never asked where I want to store a particular passkey and where I want it to be available. This is all an implicit function of a combination of factors apparently.
It's like misplacing my keys has been taken to a whole new level of abstraction :-)