system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how do big tech companies sell data? How do they turn users' data into profit & money?

How do they sell it and who buys it?
gpt
Most of the "big tech companies" you're probably thinking of (Facebook, Google) don't sell the data directly. You can't really walk up to Facebook and say "I want to buy 3 million email addresses and phone numbers, how much will that be?"

(Before someone chimes in to say that Facebook basically sold data in Cambridge Analytica or whatever, that's beside the point - giving people access to data for money is still not their primary business plan. If you think about it a little bit you'd realize that it would be a dumb business plan, because eventually you'd sell all your data, giving you nothing left to sell because it's all been leaked. Not to mention the immediate regulatory attention it would attract.)

Instead what they do is sell ads, but use the information they have to sell better ads. For example, as an advertiser you could tell Facebook that you want an ad shown to "10,000 people between the ages of 25-35 who are interested in Hiking in the US." Facebook will then show your ad to the users who it thinks matches your criteria, without necessarily revealing who those users are. Since targeted advertising like this is typically more effective than non-targeted advertising, advertisers continue spending money to run ads on Facebook and Google as opposed to other places that have less data on their users.

You can try it yourself; it's pretty easy to run ads on Google and Facebook. You can spend a couple bucks to publish an ad that will get shown to some minuscule number of users.