Custom OTP Verification System that Works with Firebase

Let’s assume you are using Firebase and all the features it supports. And now you have to deal with some kind of custom email verification to access some sort of document within the Firestore. Or let’s say you have a guest user that you need to sign in, and instead of storing them in the authentication, you are using Firestore for their entry, and you want to send a custom code to their email that they can input in the frontend and by which you can authenticate them. How would you do it?

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