Firestore, difference between startAfter and startAfterDocument

I have been reading https://firebase.flutter.dev/docs/firestore/usage/ and still do not understand yet. Documents are super confused. What are differences?

1 Isn't the code above and the code below the same?

FirebaseFirestore.instance
  .collection('users')
  .orderBy('age')
  .orderBy('company')
  .startAt([4, 'Alphabet Inc.'])
  .endAt([21, 'Google LLC'])
  .get()
  .then(...);

FirebaseFirestore.instance
  .collection('users')
  .where("age", isGreaterThanOrEqualTo: 4)
  .where("company", isGreaterThanOrEqualTo: 'Alphabet Inc.')
  .where("age", isLessThanOrEqualTo: 21)
  .where("company", isLessThanOrEqualTo: 'Google LLC')
  .orderBy('age')
  .orderBy('company')
  .get()
  .then(...);

2 I think this also has the same function as No. 1, doesn't it?

DocumentSnapshot documentSnapshot = await FirebaseFirestore.instance
      .collection('users')
      .orderBy('age')
      .orderBy('company')
      .startAt([4, 'Alphabet Inc.'])
      .endAt([21, 'Google LLC'])
      .get();

FirebaseFirestore.instance
          .collection('users')
          .orderBy('age')
          .orderBy('company')
          .startAfterDocument(documentSnapshot)
          .get();

3 https://firebase.google.com/docs/firestore/query-data/query-cursors

this is not flutter code but you can see these lines

DocumentSnapshot lastVisible = documentSnapshots.getDocuments()
                    .get(documentSnapshots.size() -1);

        // Construct a new query starting at this document,
        // get the next 25 cities.
        Query next = db.collection("cities")
                .orderBy("population")
                .startAfter(lastVisible)
                .limit(25)

It's not even startAfterDocument, but at startAfter, a raw document is being entered as a parameter. What happened? Doesn't flutter have something like this?