I deleted the weather app that came on my iPhone 7 Plus. How can I get it back?

See the following instructions from Delete built-in Apple apps on your iOS 12 or later device or Apple Watch - Apple Support

Restore a built-in app that you deleted

1. On your iOS 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.