I don’t have a iMessage app on my iPhone 10; how do I get one?

iMessages are sent using the "Messages" app (if Settings > Messages > iMessage is turned on).

The Messages app is not one of the built in Apple apps than can be deleted but with iOS 14 it can be removed from the home screen. If you have iOS 14, go to the App Library (swipe left past your last home screen). From there you can touch and hold an app and slide it left back to the home screen.

If that is not the cause, try using Search to see if that will locate it: https://support.apple.com/en-us/HT201285