Suppose I lost the phone with Google Authenticator on it. How do I restore it on another phone?
Unless you have previously backed up the QR code/secret key while initially setting up your Google Authenticator, this is not possible. Otherwise, you can contact each account's support team for help.
Alternatively, you can use another Authentication app like Authy (iOS and Android) or TOTP Authenticator (iOS and Android). These apps provide the feature to securely backup your codes and account details, so you can switch phone easily.
If you saved QR codes or secret keys in the moment of token enrollment, you should use them to restore your tokens on another phone. Otherwise, you should contact the Support team of the service which you protect with the Google Authenticator. They will help you to restore an access to your account. After this, you can issue tokens on your new phone.