The software scans your cars and hills, but doesn't call home to Apple.My understanding is that SCANNING already occurs in your iDevice library. Or else, how would the IA knows how to group all the photos with say, a bridge, cars, waterfalls, boats etc.
Now, if the software notices a unique identifier (or enough unique identifiers) of known child porn, it sends a message to Apple "hey, better have a real person take a look at these."