Yet third party YT apps skip these annoying video parts. Premium definitely feels like a huge step down if you suddenly see ads in every second video and you are not used to.
Sponsored content is regulated by Youtube's content policy like everything else on the platform, so it is in fact controlled by Youtube. The same way, the Youtuber's speech itself is controlled by Youtube, as shown by the recent “demonetize all the videos where people curse” (which generated much drama in the French Youtuber community, for cultural reasons).
