There were two different technologies. One was client-side scanning for known CSAM, which created a huge backlash and is described here: https://educatedguesswork.org/posts/apple-csam-intro/

The other is detection of images that may contain nudity, whether sent or received, when the owner/admin/parent enables the feature. It is relatively uncontroversial and is described here: https://support.apple.com/en-us/105069

That is what they actually deployed. They were planning on performing client-side scanning of all images uploaded to iCloud for CSAM and reporting it to the authorities, but backpedaled after public push-back.

The original proposal was to use a visual hash designed to identify known bad CSAM images even if cropped or otherwise edited. Your computer would scan all your stuff for these images and report you to apple who would report you to the cops. This presented a number of issues.

Accidental false positives could lead to horrific outcomes up to and including oh look bob got shot by the cops for resisting.

It was possible to produce apparently matching innocuous images and then poison people's machines with them.Oops did you click on that picture of a tree have fun with the cops. Like an advanced form of swatting.

Although inspired by a desire to find CSAM Apple could be forced to scan for ANYTHING by repressive regimes including America and China.

Although initially targeting images client side scanning of messages is a pretty obvious next step. Again obvious good motivation exists and is completely justifiable who doesn't want to stop the next mass shooting or terrorist attack... and then we can basically use it to find people critical of the regime. Do remember we are presently prosecuting a political figure for a picture of sea shells and a guy in texas is rotting in prison for distributing political literature.