Apex domains cannot have CNAME records.
Question and response
Question
dig youtube.com. NS
Answer
youtube.com. 2 NS ns4.google.com.
youtube.com. 2 NS ns1.google.com.
youtube.com. 2 NS ns2.google.com.
youtube.com. 2 NS ns3.google.com.
Authority
The response has no authority records.
Additional
ns1.google.com. 2 A 216.239.32.10
ns2.google.com. 2 A 216.239.34.10
ns3.google.com. 2 A 216.239.36.10
ns4.google.com. 2 A 216.239.38.10
ns1.google.com. 2 AAAA 2001:4860:4802:32:0:0:0:a
ns2.google.com. 2 AAAA 2001:4860:4802:34:0:0:0:a
ns3.google.com. 2 AAAA 2001:4860:4802:36:0:0:0:a
ns4.google.com. 2 AAAA 2001:4860:4802:38:0:0:0:a
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0