[brnt]

A useful keyword is psychographics, but you'll find that much research is privatized (comes from media/software companies), and that some of the meat seems to be not publicized. Which makes sense, it's what gives them an edge over the competition. The Cambridge Analytica dossier is of course a good way to find references.

In terms of the why, and the who's paying, Dark Money (Mayer) and Democracy in Chains (MacLean) touch on it: psychographics is huge for anyone doing manipulation, not just commercial ads, also political ones. Probably even moreso.