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You asked who owns the data, not what's valuable. If you are habitually located in the European Union, you have the right to access, rectify, download or erase your information, as well as the right to restrict and object to certain processing of your information. That sounds like ownership to me. I'm no lawyer but lookup GDPR laws and how that relates to the processing of 'your data' and feel free to be a test case if you believe they're acting outside that set of laws.
That's not how that works. My data itself isn't really the valuable bit, the conclusions drawn from it is. My data is also never 'taken' from me, so on that level the ownership is irrelevant if that doesn't include control over how it's used. And once they've used it there is no way to undo that - and they're pretty clear that they will use it for all kinds of stuff.