On frequency, very little. They can demonetize a video, but that also pulls it out of the recommendation algorithm, so they will only be shown on the subscription tabs of people subscribed to you. No new eyes. I think you used to be able to pick how many mid rolls, but they then changed it to either mid-rolls on or off. Monetization forces a pre-video ad. No say on the number of ads for either. So YouTube can insert as many as they think they can get away with. There’s also stories of people who had their videos demonetized for no reason, or people who had mid-rolls off and YT decided to show them anyways, and people who had monetization off but YT decided to show ads and monetize the video, because they can do anything they want and there’s literally nothing anyone can do to influence them other than whine and threaten to leave the site.

As for the content, almost zero. It’s all an automated bidding algorithm. Ad buyers place bets on how much money they want to spend per month/week/day on their ads, and choose a set of tags of the kind of people they want the ad to be shown to. Then the creators can vet some tags for kinds of ads, it’s a black-list white-list system. But anything not explicitly forbidden by the channel owner is game. They’re also super nebulous and unhelpful labels, you can’t even remotely predict which ads will be shown before your channels. When it’s time to show an ad, the algorithm brings in all the factors together and runs a bidding to determine which ad will be shown and automatically charges ad costs and revenue share.

The cherry on top, you can buy other people’s videos as ads. As in you can pay to YT, to show either your own or someone’s else videos before other people videos. Whole videos!