Flutter How to Link Multiple Auth Providers to an Firebase Account?

I am using firebase in my flutter application. In my app user can sign in or register using 3 ways.

1. Email id and password.
2. Google
3. Facebook.

This is my Firebase setting.

This is my user collection.

Now my doubt is that how do I link multiple auth providers for a user having same email id ?

I am storing user's info into User collection with Uid. If I enable multiple account to multiple providers how can I store user's data in same document ?

I have search a lot on google but didn't find proper solution.

For registration with email ID I am using this.

  final newUser = await _auth.createUserWithEmailAndPassword(
          email: widget.email, password: widget.pass);

Then I am storing user's data into User collection. (In this I have one more form, from where I am getting other data).

Google sign in code

  Future<User> signInWithGoogle() async {
    // Trigger the authentication flow
    final GoogleSignInAccount googleUser = await _googleSignIn.signIn();

    // Obtain the auth details from the request
    final GoogleSignInAuthentication googleAuth = await googleUser.authentication;

    // Create a new credential
    final GoogleAuthCredential credential = GoogleAuthProvider.credential(
      accessToken: googleAuth.accessToken,
      idToken: googleAuth.idToken,
    );

    // Once signed in, return the UserCredential
    final UserCredential authResult = await _firebaseAuth.signInWithCredential(credential);
    final User user = authResult.user;

    print('authResult');
    print(authResult);

    return user;
  }