They absolutely inundated YouTube with ads for Stadia for a couple of months, to the point that it felt like half the ads I saw were Stadia ads. So that’s something, at least?
It's not zero, because Chromecast doesn't block ads and that's the natural fit for Stadia. But it's definitely not 100% either.
How would an ad reach me? Youtubers? Can Google use its "contractors" to promote its own product? That just feels weird.