I completely erased my Mac and installed Sonoma. But 1Password information was not erased?

Hello @activeinactive! 👋

If you've already added your 1Password account to one of your other Apple devices (such as a Mac or iPad) then 1Password would have stored an encrypted version of your Secret Key in the Keychain which is securely synced to all of your Apple devices if iCloud Keychain is turned on. The next time that you need to add your 1Password account to another device you'll only be asked for your account password since the 1Password app will retrieve your Secret Key from the iCloud Keychain.

This process not only safely and securely backs up your Secret Key but saves you from having to type it into all of your devices. You can read more here: About your Secret Key

Your account information is downloaded back to the wiped Mac after you login to your Apple account. Let me know if you have any questions.

-Dave