Regarding calibration & diagnostic software for mobile devices

When repairing mobile devices whether it be iPhone or Android, Is there diagnostic and calibration software available? Or, who do I need to sleep with to get access to such tools? It seems to be a better kept secret than the gunman on the grassy knoll.

I understand devices can sometimes run self diagnostic but that function is of no use when it comes to issues like broken screens and components disabled by software. Otherwise, how can a technician get around this problem without this valuable and essential tool?

I'm not a witch doctor.