I am a bit skeptical that they don't try to monetize Premium users as well -- I mean invisibly, by selling our viewing history, timing of when we watch stuff etc. But I can't do anything about that and I openly admit all my efforts to block YouTube ads on an iPhone have failed so I eventually gave up and just bought Premium.
And to be fair, I don't regret it, because I have another consideration -- I can easily get some nice Xiaomi phone and root it and install various blockers and alternative YouTube frontends... BUT... my Google account is very valuable, a lot is tied to it and Google has become a bit trigger-happy banning and deleting accounts. So I am not willing to risk it. Hence I paid up.
My understanding is that Google does not sell user data to 3rd parties. It's much more profitable for Google to sell targeted ads that indirectly use that data (without giving the data away), and also avoids the potential legal and PR ramifications of selling user data, especially since users trust Google with a lot of their personal data.
They might use your viewing history to target ads in other places. And they definitely use aggregate/anonymized viewing patterns to suggest similar videos people might like, but I don't see an issue with that.
And to be fair, I don't regret it, because I have another consideration -- I can easily get some nice Xiaomi phone and root it and install various blockers and alternative YouTube frontends... BUT... my Google account is very valuable, a lot is tied to it and Google has become a bit trigger-happy banning and deleting accounts. So I am not willing to risk it. Hence I paid up.