[majormajor]

I never saw much at all in terms of real, taste-level, user-based personalization beyond "keep hammering them with this one thing they clicked once" retargeting.

Everything is is by aggregate. So they can put you in a bucket with a bunch of other people with a similar footprint, and if 75% of people in that bucket convert when being shown that one you hate, you get to see it too.

Low-level optimization beyond that was rare at the places I worked.

"Sneaky but obvious" was way more common. And the browsers and platform vendors kept making it harder to do certain things, so it was a constant rat race to maintain the same level of browser tracking.