Can they go from the 18 Pro Max to a Windows 11 device? Or to an Android device? Genuinely curious about the portability.

I don't think Apple bothered to implement passkey support in their Windows iCloud password implementation yet.

You can log in by scanning a QR code with your phone, though.

Or you can use a different passkey provider that does work cross-platform. You don't have to use the one built into your phone.