how do i find passcode app on iphone because i want to secure my messeges

Theres no Application for "Passwords" but there is a setting where you can see your stored passwords. Settings > Passwords. It will ask for Face/Touch ID or Password to access these stored passwords.

there is a shortcut where you can make your phone go to the Lock Screen when opening iMessage(This will work if you have a iPhone Passcode enabled). Shortcuts > Automation > +(top right) > Create Personal Automation > App > choose the app (in this case Messages) > Next > *Search for: "Lock the screen" and tap on it* > Next > *toggle the "Ask Before Running" OFF* > Done. Now anytime anyone opens your iMessage, it will force anyone out to the Lock Screen.

The messages app is not individually lockable. Set a passcode lock for the phone.