How do I get weather app back on my iPhone

Apps can be removed from the Home Screen without being deleted. 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.

Also note the Search window at the top of the App Library.

If it isn't in the App Library, see the following instructions from Delete built-in Apple apps from the Home Screen on your iOS 14 or later 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.
3. Tap the Redownload button .
4. Wait for the app to restore, then open it from your Home Screen.

If you completely deleted it, find it in the App Store App and download it again. Link-> https://apps.apple.com/us/app/weather/id1069513131

If the entry in the App Store app has an "Open" button instead of a cloud icon, then it has not been deleted. Look for it in the App Library and drag it back into a Home Screen. Link-> Organize the Home Screen and App Library on your iPhone - Apple Support