[guraf]

You're right, YouTube ads are notorious for being JavaScript heavy and served by a third party.

[jefftk]

I can't tell if you're being sarcastic? YouTube ads are served by Google, just like the rest of YouTube. You can verify this by looking where your network requests go in devtools.

(I used to work on ads at Google, but in a different area and don't know anything about YouTube beyond what I can see in devtools.)