I know it's a old thread. But I think it's very important to understand how DNS is working. Especially all the people who are using Pi-hole.
The administration of the Domain Name System (DNS) is structured in a hierarchy using different managed areas or “zones”, with the root zone at the very top of that hierarchy. Root servers are DNS nameservers that operate in the root zone.
These servers can directly answer queries for records stored or cached within the root zone, and they can also refer other requests to the appropriate Top Level Domain (TLD) server. The TLD servers are the DNS server group one step below root servers in the DNS hierarchy, and they are an integral part of resolving DNS queries.
Letter 'A' root server is called a.root-servers.net, with IP 198.41.0.4 (and 2001:503:ba3e::2:30 in IPV6). This is a valid server and operated by Verisign from the US.
See also: Root name server - Wikipedia
Limitations in the original architecture of DNS require there to be a maximum of 13 server addresses in the root zone. In the early days of the Internet, there was only one server for each of the 13 IP addresses. So that is why you only see 13 root name servers displayed on that wiki page ('A' until the letter 'M'). Using anycasted there are more servers, but one way or another there are 13 IPs as root name servers. And most of them are in the US.
Since the DNS root zone is at the top of the DNS hierarchy, recursive resolvers cannot be directed to them in a DNS lookup. Because of this, every DNS resolver has a list of the 13 IP root server addresses built into its software.