Each site is different, but a site that works with Google Authenticator (or similar apps like Authy) will have an option to scan the QR code when setting up 2FA. You then open Google Authenticator ...

The easiest way to set up two-factor authentication for a website is to scan a QR code with the app. A site that supports 2FA via an authenticator app should generate the necessary code for this ...

You can either scan a QR code using the camera on your smartphone or manually enter the code. When you see a QR code on the laptop screen, open the Google Authenticator app on your phone.