Whitelisted users can register and login

When using register/login with Google on Firebase, I want only users that I have previously authorized to register.

Is there an option to whitelist in Firebase or “only the people you specify can register”?

You ccould create a tag in firebase for example authorized_users and save a list with approved users or emails. When someone is trying to register check if is in list and if yes proceed registration otherwise show a message you must be authorized to register

Use the email verification method ?

UID’s are registered. How do I get it checked when logging in? Should I use ‘if’?

Can you show it as a block and help?

Please can someone show me how to check after login