I've done iOS/Mac app development — Apple doesn't "sell" data to me, but they absolutely provide me with extensive user tracking data for free (well, for $99 per year, but that's effectively free). As far as I know they also provide data to other third parties, such as in the news app But app developers is the big one.
The data is anonymised, but I assure you it's very detailed. Detailed enough that some companies probably cross reference it with other tracking and are able to link the data they get from Apple to real people.
Thankfully the tracking is opt-in, but users are forced to make a choice and encouraged to enable tracking and I'd argue they really aren't being educated properly on what they're handing over before making a decision. I can't really blame Apple for that, who wants to spend hours learning how Apple's tracking methods work? But it is a fact that Apple does collect a lot of data and they do share it.
Personally I have spent hours doing that research and I'm not OK with what they track — I opt out. And while my own software does have some tracking, it's a lot less detailed than the tracking Apple does. It's just basic analytics (roughly how many users do I have and what country are they from?) and crash reporting which is (thankfully) rare with my software and therefore useless for any invasive tracking. The vast majority of people using my apps never experience a crash (and that's only possible because I track crashes).