How do I add a passkey (like a yubi key) to my apple account?

See the information in: About Security Keys for Apple Account - Apple Support

Also note that passkeys are different than security keys. See: About the security of passkeys - Apple Support

hi,

Recovering or Accessing your data in iCloud

If you back up to iCloud, and you have a new device/replacement device, you can restore from your iCloud backup when you set up that device. 

If you no longer have an Apple device, or a Windows PC compatible with iCloud for Windows, then you can access some of your iCloud data on iCloud.com. 

Issues Signing in with your Apple Account

If you are having issues with signing in with your Apple Account, there are a few ways you can regain access to the account so you can sign in. 

If you know your Apple Account password, but you no longer have a trusted device, and you still have the same trusted phone number: When signing in, it sends a two-factor authentication code to the trusted device by default. Select the option of “Don’t have any of your devices” or “Didn’t receive verification code > “can’t get to my devices” then select the option “text me.” This will send the verification code to your trusted phone number so you can sign in as normal. 

Note that if you no longer have access to your trusted phone number either (such as if your phone was lost or stolen) then I would recommend contacting your carrier and having them issue a replacement SIM card you can use temporarily in another phone (does not have to be a iPhone). Once you get the replacement SIM card, have someone text you to confirm you can receive text messages as normal. If you received the message, then at that point select the option “text me”. You should be able to log in as normal. 

For any other issue with signing into your Apple Account, use these articles:

• If you forgot your Apple Account password
• How to use account recovery when you can’t reset your Apple Account password
• About trusted phone numbers and trusted devices for Apple Account 

Using Security Keys

If you add a security key to your Apple Account, you will only need your Apple Account password and the security key to sign in. The security key takes the place of the verification code that is usually sent to either your trusted device or trusted phone number. This article should help: About Security Keys for Apple Account 

Apple Jargon:

• Passkey: identity and password and two-factor authentication, combined. Increasingly common. Probably not what you want. (I also don’t know if Apple is even allowing passkeys for an Apple Account second factor. Yet?)
• Password: a secret you know, used to gain access. Used with a trusted device, in some contexts. What you’re using now.
• Passcode: used to decrypt the user data stored on an iPhone or iPad.
• Security Key: a hardware second factor for two-factor authentication. What you’re asking about.
• Recovery Key: a 28-character code used to regain access into an Apple Account. Disables all other reset mechanisms. Lose it and your password and a trusted device, and you’re permanently locked out permanently permanent. Likely not appropriate here, but will be discussed in related contexts.
• Recovery Contact: What you probably want, here.

Passkeys and passwords can be stored in iCloud Keychain, when that is enabled, which avoids issues with credentials loss when a device is lost or stolen. (Can be a problem when the device passcode is known.)

Security keys entirely replace the second factor for the Apple Account two-factor authentication.

Usual here for many people would be establishing a recovery contact, and/or a new eSIM from your carrier, or another trusted phone number, or a combination, not a security key. Security keys will work for what you want too, but you’ll want enough spares configured for the inevitable loss of keys. Probably also want the USB-C and NFC key combo, if you decide to actually use security keys.

Probably the best approach for most of us: Set up an account recovery contact - Apple Support

More background, and more details: Better Securing Your Data, and Apple Acco… - Apple Community