> If that's a secret then how would they find out?

You don't. I've worked in data acquisition in the past, both buying data and selling it. Sometimes as the original source of truth and sometimes as a middleman that does data cleaning, standardization, appending (from other sources), then selling the derived product downstream.

Companies in that space guard their upstream sources quite heavily, because they don't want to be cut out of the process. You won't find a centralized list of independent data feeds and providers specifically because of that. In one scenario, we were dealing with a substantial rate increase from one supplier. We spent time attempting to source an alternate supplier of that particular type of data, and could only find sources that were several months more stale than we were currently getting (i.e. these people were getting the feed several hops after we were). In the end we paid the rate increase because we couldn't find an alternate source that was as close to the original data provider as our current source. And without knowing who the original data provider was, we couldn't go around our supplier.

The lack of a centralized directory isn't just done to make things opaque for end users, it's done to make things opaque for business competitors as well. It's an industry that's very, very reliant on networking and introductions.

Edited to add: You're also asking a lot of people in here to name specific companies even if they can't give you huge lists. This space is super heavy on NDAs (and trigger happy on enforcing them). If you've actually worked in it, there's simply no way you're able to name drop legally.