I always hear about selling data. What is all this data worth? If I had an etsy shop or a chess site, do companies want to buy this data? and for how much and for what? Credit-score modelling?

Not in the UK or EU: they'd have to have your explicit consent to be legal.

good point - gdpr.

There are numerous data aggregators and data brokers out there. If your data is online (e.g. LinkedIn), they have it. If you've ever given data to a site that trades data with a brokerage, they have it. My company worked with one of these data aggregators and they offered us half-cost pricing if we shared our user data with them, which we declined to do, but I'm guessing many sites take the deal.

Point being, I can just about guarantee that your data is in these systems. It's probably being used for targeted advertising, recruiting tools, etc. And I wouldn't be surprised if half the data tied to your name is wrong (it was for me when I looked myself up).

good point.

tho, i was just thinking about a typical fortune 5000 company -- e.g. a FAANG, etc.

they have their big, elaborate HR ATS systems - like iCIMS - presumably the data they collect are for any number of things - but is the info sold, to a data broker or advertisers or marketers or etc.?

There is a good chance the candidate sourcing tools they used are plugged into the data brokers.