Internet activity including Safari browsing, all dns queries and all insecure http connections.
Yeah, I know Apple claims that they cannot "see" the data as it is funnelled through some other third-party - CloudFlare, Akamai etc. ... would Apple be willing to reveal the terms of the contract they have with these third-parties? How can anyone be sure Apple hasn't outsourced datamining or made another PRISM like deal with the US government to provide their users data, but through a third-party? We'll never know because there are no US laws and regulations preventing Apple from doing so.
(And when it comes to privacy that's the key thing - in the absence of laws and regulations, it is incredibly foolish to believe BigTech's word. They exist only to make money, which is their right. Hiding the truth, and exaggerating through marketing is an established practice in their world.)
Another thing to note is that BigTech are interested in promoting these kind of technology only to become gatekeepers of collecting and mining users data. Currently, some ISP's collect and store their users history. And make money from selling it. In effect, these ISP's are potential competitors to ad companies. Techs like these (https://blog.cloudflare.com/oblivious-dns/) are designed to disrupt these kind of data collection from the ISPs and undermine their business of selling data. With techs like these, BigTech now again get the upper hand in collecting, mining and monetising user data.
It's bad for distributed internet as a whole (where users have more control) because now BigTech are forcing themselves to be another intermediary, another step between accessing the data we users want.
Politically also, such techs are designed to tighten the grip of US / BigTech on the internet over that of a country's government. That is a big no-no for any democracy as a citizen loses the power to question its government's policy on the internet (including commercial and privacy laws related to it that a government may not be able to enforce because of such tech). It also weakens a country's security surveillance apparatus. Yeah, I know 'National Security' is a bogey that is used by all governments to try and curtail our rights. But would you really prefer that the usage of some of your rights be indirectly dictated by the whims of foreign BigTech or your democracy?