Enabled Device Detection on your internal interface(s), so you can easily track where your lan devices are connecting to/with. Then under FortiView, choose a device from Sources then drill down to that device's sessions - have that device access said sites to see what is getting through.
Though keep in mind that full deep packet inspection is needed if you want the Fortigate to inspect HTTPS traffic. If you are only using certification inspection then you likely only see the CN or SAN. Youtube uses Google's wildcard security certificate, so all you may see is *.google.com in the url filter logs.
Edit: See also Google's QUIC protocol and how it can affect web filtering: https://www.fastvue.co/fastvue/blog/googles-quic-protocols-security-and-reporting-implications/
