Log in to a website that supports passkeys, go to the account security page, press add passkey. Your browser looks for connected security keys to add the passkey to or if you have an extension installed, it might start your password manager. in the security key case; It asks for your pin and it's done. If you use a password manager to store it, it will ask you where to put it. Once done; you can repeat and add any number of passkeys to your security keys or database. You should do at least two, for redundancy.

I would not tie my passkey to windows/ios/android or whatever. I won't let a third party manage or sync them. Only on a security key I own or my password database.