[which]

MTProto is pretty much transport layer encryption. After the MTProto decryption occurs, Telegram servers still ultimately receive your unencrypted message encoded as a https://core.telegram.org/method/messages.sendMessage to send it to the recipient and to store it. How do you think it is possible that you can sign in on a new device and get all of your old messages? There's a reason Signal can't do that.

[gloosx]

MTProto is not pretty much transport layer encryption, Telegram servers receive messages encrypted with an auth_key which is created during registration directly on the device and never exchanged via network. When you sign in on a new device, you have to communicate the keys with your other devices, and there is also a second-factor user defined recovery password in place, which is not stored or known to the telegram servers. If you loose your permament session you may be locked out of your account and data forever. Everything is documented at the links, audited and verified, and everything is possible - you can just read how it works.