I don't want people who are using my iPhone getting into my Telegram chats, can I lock them with a PIN?
If you are using Telegram's Secret Chats for the end-to-end encryption it provides, then you probably also want to guard your chats from anybody who might pick up your phone. You can do this by opening the Telegram app and heading to Settings -> Privacy and Security -> Passcode & Face ID and tapping Turn Passcode On. You'll then be asked to enter a six digit PIN and confirm it.
Now you can choose to turn on Face ID and set an auto-lock time if you wish.