The material was allegedly stored behind password protection on his phone(s) but also on Mega and on Telegram, where Herrera is said to have "created his own public Telegram group to store his CSAM." He also joined "multiple CSAM-related Enigma groups" and frequented dark websites with taglines like "The Only Child Porn Site you need!"

My guess would honestly be Telegram. For starters, they aren't end-to-end encrypted by default, you have to turn it on. The only end-to-end encryption that Telegram offers is their "secret chats" which are only available between two users. Groups are not encrypted.