I really don’t understand the goal with vague statements like this that can’t even provide even the slightest hint of specifics. What specific data? Even a single example would make this anecdote useful.
A lot of people talk in broad strokes because if you use their marketing platforms you can see exactly what data is being mined. You can posit their ideological, political, and personal stances. Their friends, family, and pay more to reach people that are shown to influence them. You can choose their region, their income, their habits, hobbies, and kinks.
You can quickly create an account and look at their self-serve ads. There's no reason why anyone needs to try and "guess" what these tracking tools can do. You can just go to the endpoint of that collected data and just see what you can target.
It's better now but before you could do EVEN more. But better in the sense that someone who needs a limb amputating to stop gangrene setting in is better.
Additionally, there are different means to the same end, so being vague is keeping the discussion focused by keeping it about the general practice instead offering details that could easily derail into unproductive commentary. The ad firms probably move things around all the time, but the gist of it is, if you browser requests a resource from a server with a little metadata, god knows what’s being done with that from there.
The ubiquity of user tracking is extremely useful yet culturally absurd. Now that’s ambiguous ;)
Nefarious is defined as wicked or criminal. My usage was specific to the latter definition and not the former. I said nefarious and chose the definition the implies “illegal” but instead of having a conversation on the topic you chose to pick at specific word definitions.