Apple includes a customized version of OpenSSH. From what I recall from the last time I looked at it, the changes were mostly integrating the key retrieval mechanisms with the rest of macOS. For example, Apple's ssh-add can store key passphrase in Keychain with the -K option, and then later access those passphrase with the -A flag.
Those stored key passphrases are visible with the Keychain Access application, Kind: "application password", name: "SSH: /full/path/to/key", in the login & iCloud keychains.
