It's not trained. International law enforcement maintains a database of all known CP images, each image contains a specific and unique hash value created by an algorithm. Every single duplicate of an image shares the exact same hash value. The iPhone is doing all this scanning on-device, not connecting to anything. The only way apple will ever know about or see anything is if hash value of an image on a users phone exactly match that of an image on law enforcement databases.

I would be extremely unlikely to get a false positive and an airdrop troll would literally have to be using those exact CP images, in which case police should be immediately called.