[waschl]

Telegram is storing your message content in their cloud for „cloud chats“ (default), as those are not end-to-end encrypted.

Telegram‘s „secret chats“ and signal chats are end-to-end encrypted. The servers still may store metadata, and there is no way to tell if they do than either joining them or let a trusted third party verify that.

To check if e2e encrypted message content cannot be encrypted via backdoors on their servers, you need to ensure they use proven encryption schemes and the client encryption does correspond to those algorithms.