If you are in Apple world, your keys are synced between iOS and macOS (and saved in Secure Enclave, so you need TouchID/FaceID to complete flow.) In 'Google' world, you can use them between Android devices and Chrome browser.
However, if you access from unsupported app/device (e.g. use Apple Passkeys and want to access from browser on Linux machine), you can always just scan QR code with phone, and use it to log-in.
If you are in Apple world, your keys are synced between iOS and macOS (and saved in Secure Enclave, so you need TouchID/FaceID to complete flow.) In 'Google' world, you can use them between Android devices and Chrome browser.
However, if you access from unsupported app/device (e.g. use Apple Passkeys and want to access from browser on Linux machine), you can always just scan QR code with phone, and use it to log-in.
You can try it on https://www.passkeys.io