The proxy server will aggregate and anonymise the data before uploading it to upstream OnePlus servers. For example, it can strip out IP addresses. Eliminate data points which are too few and can therefore dangerous. Maybe if too few people are using an app, records about the usage of that app aren't uploaded to upstream OnePlus servers. If OnePlus wants to know how many photos an average user is taking, the intermediate server could just that information, or percentiles, not data about each individual user. These are all not possible to do on-device.
With this, we don't have to worry about what code is running on the phone, or what OnePlus servers are doing, both of us are closed-source and unavailable to us to inspect. But the intermediate server will be open-source.