whereGreaterThanOrEqualTo and whereArrayContains number of reads in firestore

It is a pretty general question but I couldn't find any document that explains this.

I use firestore as my main database for my app.

I recently implemented an autocomplete system by querying my firestore in the following 2 ways (first way to catch parts of the word and second to catch whole word if it is not at the beginning of the sentence):

db.collection( "IDS" )
        .whereGreaterThanOrEqualTo("ID", s.toString() ).whereLessThanOrEqualTo( "ID", s.toString() + "\uF7FF" ).limit(10).get()
        .addOnCompleteListener( new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                    for (QueryDocumentSnapshot document : task.getResult()) {
                        String id = document.getString( "ID" );
                        ids.add( id );
                    }

                }

            }
        } );

db.collection( "IDS" )
        .whereArrayContains("IDArray", s.toString() ).limit(10).get()
        .addOnCompleteListener( new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                    for (QueryDocumentSnapshot document : task.getResult()) {
                        List<String> idsParts = (List<String>) document.get( "IDArray" );
                        String id = TextUtils.join( " ", idsParts );
                        ids.add( id);
                    }

                }

            }
        } );

This code works well however I'm trying to understand the way it works.

Let say I have 1 million documents in my IDS collection. When I use the queries above, does it searches all of the 1 million documents and checks each one? Or more specifically, does it mean that it will count as 1 million reads or eventually since I used .limit(10) it means that only 10 reads were used?

I'm asking because this app should be published soon and I would like to figure out if the methods above will be very expensive to query.

If there is another way to optimize the query I would be happy to know.

Thank you