The New York Times wrote about a still-unreleased report from Chckpoint and the Miaan Group:

The reports, which were reviewed by The New York Times in advance of their release, say that the hackers have successfully infiltrated what were thought to be secure mobile phones and computers belonging to the targets, overcoming obstacles created by encrypted applications such as Telegram and, according to Miaan, even gaining access to information on WhatsApp. Both are popular messaging tools in Iran. The hackers also have created malware disguised as Android applications, the reports said.

It looks like the standard technique of getting the victim to open a document or application.