Already happened. They have search ads for apps; safe to assume they can tie your use of apps into that dataset. Just because their primary business model is hardware and software sales doesn't imply they don't have a hand in ads; the ROI and margins in ads are stupid high.
https://developer.apple.com/documentation/apple_search_ads/s...
That said, it's a matter of time until an Apple CEO looking for a way to juice the quarterly numbers finds a way to put that data to work...