My understanding is that when a channel's content is not monetized, no ads are displayed. So no, YouTube is not pocketing the money they would have paid him.
YT most certainly runs ads against demonetized content. They just don't share in the take with the creator. What a principled stance... "I will make even more money because you did something bad somewhere else!"
No. Since there is a lot of disagreement about this in the surrounding comments, I just watched 20m of an old Russell Brand YT video. As of a few minutes ago there were no YT ads in the video. That includes the very beginning and the end. However there were Brand narrated promotions in the video.
The video was actually much easier to watch without the blasted YT ads that normally pop up every 5 minutes. And I suppose Brand still gets paid 100% for the embedded promos he narrated and negotiated with the advertisers.
What Youtubers call "demonetization" usually refers to their monetization status changing to "limited to no ads." Even if their video is in the category of "limited" ads, the video tends to make a small fraction of what a regular video would make because there are fewer bidders for these ad slots, so from their perspective, it's basically the same as not being paid at all. In Russell Brand's case, it looks like his channel got completely demonetized, so his channel shouldn't have any ads.
There's a bit more nuance to that. Some advertisers can choose to put ads on videos that are labelled undesirable. But because there's less advertiser demand for ads on such videos, they're generally very cheap.
There was a time after I took a media class where I had insomnia and watched a lot of late night TV. I knew about targeted advertising, and the kinds of ads on late night TV at the time… oh boy. The lowest quality camerawork, dialog, and product. For a while I felt insulted, that they thought I was the target audience for this schlock. Then I realized that no, this is just the cheapest ad slots of the day, and so they are running bottom of the barrel ads during this time.
So I guess we can expect to see wannabe-Billy Mays ads on demonetized shows.
unless they changed things, last year YT made it impossible to not display ads on your videos even if you wanted to let people watch your videos without ads
I believe this is true when you are a partner and you opt out of ads for a video. But if you're not a partner yet, then YT will serve ads that you don't receive any income for.