Automatic Photos transfer from iPhone to Mac

I have an iPhone and a Mac, both running latest versions of iOS and macOS.

On the iPhone, in the Settings > Photos tab, as well as in the Preferences of the Photos.app on the Mac I have My Photo Stream enabled, as well as Shared Albums, but I have iCloud Photos disabled.

I was under the impression (as is stated under the "My Photo Stream" setting) that "Photos from other devices can be viewed in the My Photo Stream album, but are not automatically saved to your library".

However, on my Mac in the Photos app, in the general library (i.e., the very first category called "Photos" that comes before "Memories" right under the general heading "Library") I see all of the photos taken on my iPhone (since taking photos on the iPhone) without me having imported them manually. In the beginning days I actually did drag some photos manually from the iPhone to the Mac, but haven't done that for the past two years or so. And I rarely open the Photos app on the Mac anyways.

I am rather positively suprised by this. Under "My Photo Stream," as expected, only the last 30 days are shown (on both, iPhone and Mac). But I did not think older photos would automatically be added to my Mac's library.

Is this a new feature possibly? Documented somewhere?

I would love to know a bit more on how this works/is supposed to work.

Also, are the photos that synced the originals or some smaller/compressed versions of the photos?