Remarkable investigation into Telegram by IStories (in Russian):
https://www.istories.media/stories/2025/06/10/kak-telegram-svyazan-s-fsb/
English version by OCCRP:
http://www.occrp.org/en/investigation/telegram-the-fsb-and-the-man-in-the-middle
tl;dr:
👉 Telegram uses a single company with ties to the Russian FSB as their sole infrastructure provider, globally.
👉 Combined with a cleartext device identifier Telegram's protocol requires to be prepended to all encrypted messages, this allows for global surveillance of Telegram users.
I am quoted in this story.