It hasn't particularly changed production, which is where the actual abuse happens. There are still actual people abusing and filming actual children, and those can be found by the police by the same old-fashioned methods they've always had available. (Plus many new ones that don't violate everyone's civil liberties or destroy the security of every networked device.)

They can find those materials the same way abusers work their way into communities to have access to them in the first place.

The increased scale only means they need more people working on it.