type here
bool
create_and_insert_doc (mongoc_client_session_t *session,
                       void *ctx,
                       bson_t **reply, /* out param for our server reply */
                       bson_error_t *error)
{
   /*
    * mongoc_collection_insert_one requires an uninitialized, stack-allocated
    * bson_t to receive the update result
    */
   bson_t local_reply;
   bson_t *doc = NULL;
   ctx_t *data = NULL;
   bool retval;


   for (int i = 0; i < 1000; i++) {
      doc = BCON_NEW ("new", BCON_INT32 (2), "val", BCON_INT32 (2));
      printf ("Running the user-defined callback in a newly created transaction...n");
      data = (ctx_t *) ctx;
      retval = mongoc_collection_insert_one (data->collection, doc, data->insert_opts, &local_reply, error);
      {
         sleep(0.5);
      }  
   }
   printf ("sleep end...n");
   /*
    * To return to the mongoc_client_session_with_transaction() method, set
    * *reply to a new copy of our local_reply before destroying it.
    */
   *reply = bson_copy (&local_reply);
   bson_destroy (&local_reply);

   bson_destroy (doc);
   return retval;
}

mongoc_client_session_with_transaction (session, &create_and_insert_doc, NULL, &ctx, &reply, &error);

When I make Transaction using mongoc driver above. And run the code which runs about 5 seconds just inserting documents.
Other operation like insert, update, create index are not blocked. why is that?

I know mongodb use intention lock, Shared lock, exclusive lock. So If I make transaction which just insert single documents. intent exclusive lock is obtained for database and collection. exclusive lock for the document.

I think I am missing or misunderstanding something. Thanks for reading.

I ran test code. checked db.currentOp(). ran insert, update, create index while running tranjection.