How to get data of a document from another collection having its document id?

Question

Hello im new to firebase.

I have 3 collections

Users: - userid

Providers: - Providerid - providerName - providerEmail

FavoriteProvider: - docId(userId+providerId) - userId - providerId

So, i have succeed put a favorite provider for a user and the user can have many favorite providers.

But how to get a stream list of favorite providers of that user in one list.

I tried using this code

Stream<List<SerProviders>> get favoriteProviders{
   favoriteProvidersCollectionRef.where('userId', isEqualTo: uid).snapshots.map(providerListfromsnapshot)
}

The function providerListfromssnapshot returns a list of providers according to model a created in flutter.

List<SerProvider> providerListfromssnapshot(QuerySnapshot snapshot) {
Return snapshot.docs.map((doc){
   Var data = doc.data() as Map<String, dynamic>;
Return SerProvider(
  ProviderId: doc.id,
  ProviderName: data['providerName'] 
);}).toList();
}

But no idea how to complete.

I need the data of the provider (id and name) in that stream.

Anyone help me plzzz