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

The controversial part is having the system enabled by default with age verification required to turn it off, and having the system impact non-Apple/Google apps. The UK for example wants Apple and Google to forcibly enable nudity blocking on all devices in the UK, and they want the system to bypass app/DRM security to scan all content visible on a device.