How to get specific data from firebase with Flutter?

Question

(EDİT) I am getting error in below code, what I want to do is only get data of followed users, users are in a different collection, shared content is in a different collection, and I'm getting an error on the 4th line, I hope I explained it correctly, I would appreciate if you could help.
Errors I get:

The getter 'data' isn't defined for the type 
'Stream<QuerySnapshot<Map<String, dynamic>>>'.
Try importing the library that defines 'data', correcting the name to 
the name of an existing getter, or defining a getter or field named 
'data'.

My Code:

 stream: FirebaseFirestore.instance.collection('posts').orderBy('datePublish',descending: true).snapshots().map((event) {
      List following = [];
      var userSnap = FirebaseFirestore.instance.collection("users").snapshots();
      if(Provider.of<UserProvider>(context).getUser.following.contains(userSnap.data["uid"])){
        following.add(event);
        return following;
      }
      return following;

    }),

userSnap.data, data is not defined!