There was a customized encryption protocol built into an existing app popular with criminal figures, based on information given in a plea bargain with a criminal.
This company as well as an RCMP insider were involved in an earlier app. The archived page below seems to be some sort of security analysis of Anom comparing it to a competitor.
Likely there were compromised servers run by or monitored by law enforcement in different locations; exact technical details may be harder to find but the more straightforward (not necessarily only) option would be to exfiltrate messages from these compromised servers.
[..] FBI, AFP [Australian Federal Police], and the CHS built a master key into the existing encryption system which surreptitiously attaches to each message and enables law enforcement to decrypt and store the message as it is transmitted. A user of Anom is unaware of this capability. By design, as part of the Trojan Shield investigation, for devices located outside of the United States, an encrypted "BCC" of the message is routed to an "iBot" server located outside of the United States, where it is decrypted from the CHS's encryption code and then immediately re-encrypted with FBI encryption code. The newly encrypted message then passes to a second FBI-owned iBot server, where it is decrypted and its content available for viewing in the first instance.
According to this story
Anom's website currently has a message saying, "This domain has been seized" and that "[l]aw enforcement has been monitoring messages and attachments from the ANØM platform. A number of investigations have been initiated and are ongoing."
The goal of the new platform was to target global organized crime, drug trafficking, and money laundering organizations, regardless of where they operated, and offer an encrypted device with features sought by the organized crime networks, such as remote wipe and duress passwords, to persuade criminal networks to pivot to the device.
The FBI and the 16 other countries of the international coalition, supported by Europol and in coordination with the US Drug Enforcement Administration, then exploited the intelligence from the 27 million messages obtained and reviewed them over 18 months while Anom's criminal users discussed their criminal activities.
The way this came about was covered in an earlier
draws from an unsealed court document containing an FBI affidavit and application for a search warrant. The 2018 arrest of Phantom Secure CEO Vincent Ramos, who sold encrypted phones to criminals, helped lead to the FBI operation. "In the wake of that arrest, a confidential human source (CHS) who previously sold phones on behalf of Phantom and another firm called Sky Global, was developing their own encrypted communications product," Vice wrote.