Using subcollection from a query returned

No, this is not possible without issuing a new query. There is no method that allows that for a DocumentSnapshot.

To illustrate that, let's imagine you get a city document through a query (which you know returns only one doc):

var citiesRef = db.collection("cities");
citiesRef.where("name", "==", "Brussels").get()
  .then(function(querySnapshot) {
      if (!querySnapshot.empty) {
          var doc = querySnapshot.docs[0];
          console.log("Document data:", doc.data());
      } else {
          // doc.data() will be undefined in this case
          console.log("No such document!");
      }
  }).catch(function(error) {
      console.log("Error getting document:", error);
  });

and you know that this doc has a sub-collection for restaurants in this city. If you want to get this collection you would need to do:

var citiesRef = db.collection("cities");
citiesRef.where("name", "==", "Brussels").get()
  .then(function(querySnapshot) {
      if (!querySnapshot.empty) {
          var doc = querySnapshot.docs[0];
          var restaurantsCollRef = citiesRef.doc(doc.id).collection("restaurants");
          return restaurantsCollRef.get();    
      } else {
          throw new Error("No such document!");
      }
  })
  .then(function(querySnapshot) {
      querySnapshot.forEach(function(doc) {
         console.log(doc.id, " => ", doc.data());
      })
  }).catch(function(error) {
      console.log("Error getting document:", error);
  });

One point to be noted is that actually a document and one of its sub-collection are not, from a technical perspective, relating to each other.

Let's take an example: Imagine a doc1 document under the col1 collection

col1/doc1/

and another one subDoc1 under the subCol1 (sub-)collection

col1/doc1/subCol1/subDoc1

Actually, they just share a part of their path but nothing else. One side effect of this is that if you delete a document, its sub-collection(s) still exist.