There are ads on the app store, which I'd consider to be part of the "primary operating system", especially since it's the only way to install apps.
Not to mention constant ads for icloud. In the photos app, and even notifications from the settings app. (It's possible to turn these off, but not easy or intuitive).
After switching to Android, I haven't seen a single ad in the operating system, (I think Play Store does have ads, I just haven't got any for some reason). The closest thing is Google photos sometimes asking me to turn on backup.