Do I need the same PIN unlock code for Apple Passkeys on iPhone 16 Pro and Mac mini?

No. Your iPhone passcode and Mac login password don’t need to be the same to use passkeys.

Passkeys sync through iCloud Keychain when the devices use the same Apple Account and Passwords & Keychain is enabled.

If Face ID isn’t set up on the iPhone, you can use the iPhone’s device passcode instead. On a Mac mini without Touch ID, choose Other Options > Passkey from nearby device and use your iPhone, or use a compatible security key.

Apple explains the iPhone passkey options here: Use passkeys to sign in to websites and apps on iPhone - Apple Support

In fact, there is an important terminology distinction:

• iPhone: You unlock it with a device passcode (and optionally Face ID/Touch ID).
• Mac: You unlock/sign in with a login password (and optionally Touch ID).
• These are not the same credential, even when both devices use the same Apple Account.

Your Apple Account password is also separate from both.

So, for example

If your iPhone has: iPhone passcode: 123456

and your Mac has: Mac login password: ABCDEF

Apple does not synchronize the iPhone's passcode to the Mac or the Mac's login password to the iPhone.

The same Apple Account primarily establishes your identity and enables services such as iCloud, iCloud Keychain, Find My, etc. It doesn't make the devices' local unlock credentials identical.

And one more subtle point: Apple's "passkey" terminology normally refers to website/app authentication credentials, not the passcode used to unlock an iPhone or the password used to log in to a Mac.

Passkeys and Apple Devices

• iPhone: To access a passkey, you authenticate using the device passcode (or Face ID/Touch ID).
• Mac: To access a passkey, you authenticate using the Mac login password (or Touch ID).
• The iPhone passcode and Mac login password are separate credentials and are not synchronized.
• Your Apple Account password is separate from both.
• To have the same set of passkeys available on both your iPhone and Mac, make sure both devices are signed in with the same Apple Account and have iCloud Keychain/Passwords syncing enabled.
• This is because your iCloud Keychain is associated with your Apple Account. If the iPhone and Mac are signed in to different Apple Accounts, they will have two different sets of iCloud Keychain passkeys.

Bottom line: The credentials used to unlock each device are different, but the passkeys stored in iCloud Keychain can be shared across both devices when they use the same Apple Account.