All data sent to Telegram’s servers will be encrypted once they reach the servers
Except for "secret chat" (which are only 1-on-1 chats, have flaky client support, and require both participants to be online at the same time to initiate; in other words, they are near useless) - this is just simple at-rest storage encryption. They possess the keys to decrypt your messages (again, except for secret chats), because that is necessarily what happens when they serve those messages to recepients.