"Secret Chat" uses Telegram's (flawed) E2E protocol, so the server would only see ciphertext. A "normal" chat is stored in plaintext.
This is also why normal chats work in multi-device environments, but secret chats don't. Unlike iMessage (and I assume Signal - haven't looked at the actual protocol), they don't do anything fancy like making the sender encrypt messages with multiple public keys (one for each device the recipient owns).
This is also why normal chats work in multi-device environments, but secret chats don't. Unlike iMessage (and I assume Signal - haven't looked at the actual protocol), they don't do anything fancy like making the sender encrypt messages with multiple public keys (one for each device the recipient owns).