How does T-Mobile gather app usage data it then sells to third parties?

There's been some talk about T-Mobile selling app usage data to third parties:

https://www.adexchanger.com/mobile/t-mobile-rebrands-its-ad-biz-and-navigates-the-perilous-line-between-programmatic-and-privacy/

https://www.phonearena.com/news/t-mobile-app-insights-selling-user-data_id140978

The question is: how is this data gathered? Is it via a hidden service in the OS? Or by monitoring network traffic? The privacy policy osn't if much help in this regard:

https://www.t-mobile.com/privacy-center/our-practices/advertising-solutions-privacy-notice

Even though it claims to describe "how we collect, use, and share personal data", it does not at all answer the question of "how do they collect the data".

So... how do they collect the data?