In short: Only the most necessary.
Obviously, we need to store and process your profile pictures and answers to display your profiles and chats. We truly only need all the other data for two purposes: to ensure the app runs smoothly (for example, we need to know what type of phone you're using), and to identify errors within the app. That's it. All data generated while using the app remains on our servers (in Germany), and we don't share it with anyone else, simply because it's important to us. This is also why we don't display ads in the app, as doing so would require sharing data with advertisers. And if you delete your account, all the data we had about you is also deleted from our servers :)
And yes, we know that selling data can be very profitable. But rest assured, as long as this statement is here, it won't happen. Of course, there might come a day when we sell Blindmate to some company. In that case, this statement would likely disappear, replaced by an updated privacy policy with the headline "Data protection is important to us," which nobody reads. But for now, we don't need to do that :)