We pick i.root-servers.net, one of the 13 root servers (1), and ask it about sjl-ins1.sjl.youtube.com and learn that com is delegated to 13 name servers (2), so we ask f.gtld-servers.net where we learn that youtube.com is delegated to four name servers, including ns4.google.com (3), so we ask that one and get the following answer: