Many people log into their Mac using the same credentials (Apple ID) that give access to the Health data, and in fact Apple makes it really hard or even impossible to use it without (you can't selectively grant access, you need to use a separate Apple ID but then you lose some useful features such as universal clipboard, etc).

This is again a misinformed take. Your Mac can already get all your iPhone's data from the cloud where it is synced without viable opt-out or compartmentalization.

> Your Mac can already get all your iPhone's data from the cloud

Only if the data is available in iCloud and it is stored in files and it is not encrypted.

Otherwise data from apps like Instagram will be exposed exclusively via screen sharing.

There’s a universe in which your Mac is a locked down device like your iPhone, with a proper immutable filesystem, carefully controlled persistent state, and a strong sandbox in which the terminal, Homebrew, and apps (App Store and otherwise) can act within the sandbox but cannot do things like, say, reading your entire iMessage database.

We do not live in this universe. Consider getting a Chromebook instead if you want to be in that universe. (But then you have a tradeoff: Apple itself seems pretty good about not using your data inappropriately. Google, not so much.)

> Only if the data is available in iCloud and it is stored in files and it is not encrypted.

Health data is available in there, just to go after your example. iPhone backups are also available in there.

At no point am I being asked anything else beyond my Apple ID, password, and two-step approval on another device (such as the Mac) to set up a new iPhone and download all my data.

Thus the outcome is that the Mac indeed has everything it needs to get access to all your iCloud data. In fact, reverse-engineering how to get it directly is unnecessary work - instead, just reverse-engineer enough to capture the Apple ID password (or prompt to it - given there's still no way for the user to tell a real system dialog from one drawn by malware) and approve the 2FA prompt, get an actual, real iPhone and sign into the person's account and then extract all the data from there (via screenshots if necessary).