main.dart

void _onPressed(){
  var firebaseUser =  FirebaseAuth.instance.currentUser;
  firestoreInstance.collection("users").doc(firebaseUser.uid).set(
  {
    "name" : "john",
    "age" : 50,
    "email" : "example@example.com",
    "address" : {
      "street" : "street 24",
      "city" : "new york"
    }
  }).then((_){
    print("success!");
  });
}