Most people know which countries developed stuxnet and used compromised human agents to place the infected drives into the target system.

The sophistication of the malware required state powers and assistance of the industrial company that manufactured the control systems. Those systems are not cheap and reverse engineering the control system software required detailed knowledge of the embedded hardware and software.

From what I've read and heard about the attack in the past, they had managed to find the supplier and had confiscated some blueprints or the actual structures (nuclear reactor?) that was used and then reverse engineered it from there with the help of nuclear scientists to find vulnerabilities in it.

from what I understand, they have the same controller that was being used in the centrifuges on their desk to develop the payload the rest was just a way to get to the controllers of the centrifuge which was rather standard.

Compromise an employee.