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There are two different CSAM related features. One scans photos before they get uploaded to iCloud, other looks for indications of nudity and only works on minors' phones if their parents enabled this detection.

As far as I know, it’s only iCloud Photos.

Photos in Photos.app will get scanned if you have iCloud enabled (required to download the CSAM hash database).

Messages will be scanned for CSAM content if you are a child, then parents will get notified that they received a dickpick.

To be clear, the content being scanned in messages is not a hash check against a database. It’s a plain old nudity detection algorithm similar to what Facebook has.