If you shared admin password , to other person using your old Mac , now you upgraded to Mac OS Catalina 10.15.1 , when a new Mac is set https://support.apple.com/en-in/HT205749
and iCloud Keychain is set up https://support.apple.com/en-in/HT204085
A new iCloud Keychain verification will be created overwriting the previous , so the iCloud passwords are still secured and no one can enter , also when you erase the Mac and reinstall , every files are refreshed , a new admin account and its password can be set .
And once 2FA is activated , all security answers for Apple ID account , date of birth added are deactivated see this article https://support.apple.com/en-in/HT201303