@bot The system update feature.

Apple can push an OS update to any of their internet connected computers at any time, with the functionality of their choosing (per user custom "updates" are trivial to do as well) - as they hold the signing keys, which is all that's required for an "update" to be installed.

The user isn't ever informed as to what's in an OS update, cannot check what's really them and also can't deny an update - updates can only be deferred temporarily.

The user can't even downgrade to older OS versions after a set time period, as apple revokes the signing keys for such older versions at a certain schedule, after which makes installing such older versions impossible.

Even assuming that Apple's OS's don't currently spy of the user, Apple can at any time send an update to add spying functionality, so the only logical to assume that there's spying functionalities and worse unless you have evidence that there isn't.