They do this through a few devious practices, such as the spin doctoring discussed here. One other thing that they do is instead of the option being a check box, it is a selection between two radio buttons and you explicitly have to choose one of them. I suppose this is actually a violation of the GDPR, since it should be disabled by default.

You don't lose value from facebook at all by not opting in. "Show ads that are relevant to me," is an arrogant statement. The actual statement is "Track my browsing to show ads that Facebook believes are relevant to me." I don't use Facebook for the targeted ads, I use it to infrequently receive news about people I know spread out across the globe.

I more was curious of whether the history cleared with Clear History are primary markers for ad targeting or if other data is used, and in reality if enough users don't clear their data but have similar data fingerprints (the non-Clear History data) then Facebook could still present assumptions to target by for those who do clear their history regularly, e.g. it will be a moot point up until say 40% of people regularly are clearing their history and therefore can no longer accurately enough make those assumptions.

Not at all, the NN that does ad targeting has already been trained on the data. Deleting it will make no difference (assuming you believe they actually will delete it)