what is keychain icloud exact

There is some information about that, including the following excerpt, in https://support.apple.com/en-us/HT204085

iCloud Keychain keeps the passwords and credit card information that you save up to date only on the devices that you approve. When you turn on iCloud Keychain on an additional device, your other devices that use iCloud Keychain receive a notification requesting approval for the additional device. After you approve the additional device, your iCloud Keychain automatically begins updating on that device.

iCloud Keychain is protected with industry-standard encryption techniques on all of your devices, both in transit and in the cloud.

If you use two-factor authentication, your device is trusted when you sign in. In most cases, you won’t need to approve your device separately to turn on iCloud Keychain. Depending on your account and device setup, you might need to enter your iCloud security code, enter the passcode that you use to unlock one of your other devices, or approve from another device.