There is no "DNS hierarchy".
This is the most common misunderstanding of how the DNS works. People get it into their heads that DNS has some hierarchy of servers, all querying one another up and down a long chain. That is not how the DNS works. There's no hierarchy of ever higher servers all querying one another.
Do you understand HTTP and the WWW? Yes? Good. DNS is just like that. A set of content servers publish content taken from their back-end databases, and a set of proxy servers, that do the grunt work, sit in between those content servers and the DNS client library code that is linked into applications programs that want to use the DNS. An application performs a front-end transaction with a proxy, and the proxy does a whole load of back-end transactions with various content servers to construct the complete answer that it returns to the application. Content servers don't query other content servers, and the only time that a proxy server talks to another proxy server is when it is forwarding the entire transaction for the forwardee to perform the grunt work.
One can configure multiple proxy servers for the client library code in applications programs to send their front-end queries to. In Microsoft terminology for Windows NT, these are the (set of) "preferred" and the (set of) "alternate" (proxy) DNS servers that one configures a workstation with. These servers do not form any sort of a hierarchy. Clients pick amongst them arbitrarily, in ways that vary from client library to client library (and according to various client library bugs), and they should all thus behave identically to one another for proper operation. Usually, ISPs provide at least a pair of identically behaving proxy DNS servers for customer use, so that if one server goes down the client libraries can fall back to the other.