They are stored in your platform's password manager. So they're available on all the devices you're logged into.
If you're enrolling a new device (say you buy a new android phone) you can scan a QR code from your previous phone go log in.
If you're enrolling a new device (say you buy a new android phone) you can scan a QR code from your previous phone go log in.
If you want to use both you simply enroll both your PC and your iPhone. There's nothing stopping you from doing this. You can register multiple passkeys from different providers to the same account.
You can also log in to your PC with your iPhone by scanning a QR code. And then afterwards enroll your PC as a secondary passkey.