It does not work with a standard forwarding. All intranet servers need to enable forwarding, the internet becomes root. If it forwards it is not a root server. Root servers can forward if configured to. There is nothing special about serving the root zone that precludes the server doing this.
Root servers are only athoritative for the zones they are configured for.
If they need to get answers from zones they are not configured for the can and will both recurse and use forwarders. That being said the root servers on the Internet are configured not to recurse or to fetch glue and hence the forwarding state in mute.