That said, I checked the parent's video, watching it all the way though... and I also didn't see any ad (and I even watched it on my Chromecast, which usually triggers a lot more ads on Youtube, which is a reason why I never use it for Youtube, but only for Netflix, Disney+, etc.)
I assume that ads might be shown, but only rarely. Probably Youtube wants to run them, to have the flexibility to recoup on some of the infra costs for videos which wouldn't otherwise earn revenue (i.e. they aim net zero for non monetized videos... but this is my speculation)
