Hi, there.
I am Andrey from Russia, working as a mobile developer.
I've read your description and I'd noticed that you are looking for Flutter Expert.
I am very familiar with dart/Flutter and Google Firebase.
I had finished social app a few days ago using Firebase and Flutter.
I will send my past product over a chat.
You seem like you are asking how to integrate firebase authernication to flutter.
I can solve your problem within a hour.
Thanks.