When you log into your phone with an apple ID, they hold a record of every interaction you have using it. Download an app, song, or backup your phone using icloud? Use their health app? It’s all stored and associated with your apple ID and phone serial number as well as other device unique identifiers. They reserve the right to analyze this data in-house or 3rd party and how you use their products and services to improve them, produce new, market to, etc.