What is the difference between Keychain & Passkeys?

The two terms are related but not identical. Keychain in macOS is a system provided by Apple for storing passwords and other security information. A Passkey is an alternative method of providing security for an on-line service or account, promising to be more secure than passwords. See:

https://www.macworld.com/article/219714/how-to-manage-passwords-with-keychain-access.html

https://www.howtogeek.com/854319/what-is-a-passkey/

Regards.

A "keychain" is a collection of passwords (access keys). It can be likened to that ubiquitous jangly object hanging from the belts of so many janitorial service workers.

A "passkey" is an improved version of a single password, used for access to a single site or application or device.

See also info at the links posted above by user @varjak paw.

You're welcome. 🙂