There is no single root dns server there is technically one for every possible domain. Most the core central ones only accept connections from other DNS servers.
Still you can pretty much use any dns that will respond to you and it will work fine. The reason a lot of people use 8.8.8.8 from google is that IP does not represent a single machine. They have a server in almost all major cities so you get fast response. If it would fail then it would just go to the next closest 8.8.8.8 ip address. Now if you are worried about google data collection you can use 4.2.2.2 what works the same way but is not in as many location. This is level3 dns.