I don't experience that though I have friends who use smartphones who describe it. So I think a lot of it is via javascript. I doubt every retailer, or even a significant fraction, has their backend sending that type of data to $megacorp. But maybe I'm just lucky or shop weird places or it's because I use a new email address @superkuh.com for every account sign up. Or maybe I'm just not seeing the targeted ads for my $superkuhprofile that do exist because I have almost all ads successfully blocked. Perfect is the enemy of good anyway, all mitigations help a bit. And blocking JS is a huge mitigation.