Creators have the final decision on the ads shown. I have a YouTube channel, and I always disable monetization on all my videos, so no ads are ever shown on my channel.
If you see an ad, it's because the creator decided to include one. YouTube is simply the platform that enables this choice.
They will be improving ads on the platform by removing the creator's control over them.
> optimizing creator revenue and taking the guesswork out of which ad formats to use by removing individual ad controls for pre-roll, post-roll, skippable, and non-skippable ads on newly uploaded videos.
https://support.google.com/youtube/thread/233723152/simplify...
Anyway, at present it's possible to disable them if you have reached the monetization requirements on the channel, like 1000 subscribers and some amount of hours viewed.
But after the channel satisfy such requirements, like 1000 subscribers, the creator gets control of the monetization and they can choose.
For any major channel you watch, it's the creator decision that you see ads or not.
One of the videos was hit with an automated DMCA takedown, one was hit with a "mature content" takedown, and the remaining 3 have ads on them now.
This applies to videos with no copyrighted content. If you use any excerpts or music that someone else claims, anything might happen to your video regarding ads.