"Telegram provides a feature called "Links previews" that's available and on by default in not encrypted chats, anyways if you use a "Secret chat", Telegram app

"Telegram provides a feature called "Links previews" that's available and on by default in not encrypted chats, anyways if you use a "Secret chat", Telegram app ask to you if you want to use "links previews" adversiting that it previews are done in the server side. How can they know what links are you writing? Can they read your messages still if you are using "Secret chats"? (Not sure but is a edge case)." They probably do pattern matching on the app, why would they need to send anything to the servers for content previews?