I would say where personalized content becomes dangerous is the TikTok argument. So, for the unaware, there are actually two TikTok applications: the "China only" application and the international one which most of you are using. Both are operated by the same company (Bytedance) yet produce very different results
according to researchers who have examined the app.This idea that we flock to things that disengage us while affirming our world view without challenging us sets a dangerous precedent. It reinforces the "dunning-kruger effect" wherein we know a lot of things but very little about any one of them, leading us to make really stupid assumptions about the world we live in.
As to how that information is curated to begin with, it's been a known fact that everything you do is timestamped and geolocated - even if you aren't signed into any particular service. The reason for this is because you are not alone when you browse. Your workstation or terminal has its own ID (as in, its IP address), so as your workstation accesses websites, it has to ping multiple third-party servers to do so. This creates a digital trail and is why VPNs became popular. (That's its own complex topic that I refuse to go into.)
The bottom line: Personalized targeting should be an assistive tool, but it shouldn't be used to run your life.