You are able to share an Apple passkey to any nearby Apple device at any time using AirDrop. Passkeys can also be used cross-platform during sign in via an NFC/Bluetooth handshake initiated by QR code.
Additionally, passkeys are just a synced-via-cloud implementation of FIDO2, an open standard that has other implementations you may feel more comfortable using.
For someone who requires being able to sign in to, say, GitHub from multiple different operating systems or platforms, you have a few options.
1. Use a passkey on your primary device, say an iPhone. You can still sign in to GitHub on a Windows computer or Android phone but you must have your iPhone with you. During sign in, there is an option to show a QR code on the Windows/Android, which you will point your iPhone at, and the two devices will do a secure handshake to sign you in. This is probably the worst option from a UX standpoint if you sign in on lots of devices that are not your primary.
2. Use a physical security key to store a FIDO2 key instead of a passkey. These devices are inherently cross-platform. Remember, a passkey is just a type of FIDO2 key. No one is forcing you to store it in the cloud. You can buy something like the YubiKey 5C NFC to store your keys completely offline and under your own control. The tradeoff is you will need to have it with you and you will need to plug it in every time you create an account or sign in.
3. Add multiple passkeys to your GitHub account, one for each platform you want to be able to sign in on. Unlike passwords, where an account generally only has one password at a time, it’s normal and even recommended to have at least one backup FIDO2/passkey registered with an account.
And of course these aren’t mutually exclusive, you can mix and match these techniques, perhaps depending on how important the account is or how/where you typically access it. Maybe you only use a single passkey on your primary device for your bedtime social media scrolling, but use a passkey with a backup FIDO2 security key on GitHub.