If I deleted the messages app how do I get it back?

Messages isn't an app you can actually delete. Make sure you haven't misplaced it. Swipe down on the home screen and search for it.

Organize the Home Screen and App Library on your iPhone [iOS14] - https://support.apple.com/HT211345

Delete [and restore] built-in Apple apps on your [iOS] device or Apple Watch - including a list of which ones can be deleted.

https://support.apple.com/HT208094

Some apps cannot be deleted but may be missing if disabled with Parental Restrictions or ScreenTime as in this topic: https://discussions.apple.com/message/33168221#message33168221

Use parental controls on your child's iPhone, iPad, and iPod touch - https://support.apple.com/HT201304

Screen Time - https://support.apple.com/HT208982

Settings > Screentime > Content & Privacy Restrictions > iTunes and App Purchases

See the following from Delete built-in Apple apps from the Home screen on your iOS 14 or iPadOS device or Apple Watch - Apple Support

Restore a built-in app that you deleted

1. On your iOS or iPadOS device, go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
3. Tap  to restore the app.
4. Wait for the app to restore, then open it from your Home screen.

Also, note that with iOS 14 apps it can be removed from the home screen without being deleted. 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.