It is client side scanning embedded in the apps themselves. Each app will have to deploy their own mechanism to intercept the messages. This is not (yet) an OS level scan so there is no AI bot watching your moves on your device yet. Furthermore the AI part will run on their own servers, not on the device.

Precisely, the way it has been described, is when you hit the send button, it will the send the message in clear text to the authorities and then send the encrypted message to the recipient, hence the stupid narrative from the proponents of Chat control that it does not break encryption because it was never encrypted in the first place.