the application keychain has disappeared on my iMac. and iCloud
how do I get the keychain app on my iMac and my iCloud
If the Keychain Access app is no longer present on your Mac then reinstalling macOS is justified: How to reinstall macOS from macOS Recovery - Apple Support.
It is not normally possible to delete the Keychain Access app, so if it no longer present on your Mac the reason for that needs to be determined and addressed.