See the following from Delete built-in Apple apps from your iPhone, iPad, or Apple Watch - Apple Support
Restore a built-in app that you deleted
- On your iPhone, iPad, or Apple Watch, go to the App Store.
- Search for the app. Make sure that you use the exact name of the app.
- Tap the Redownload button
. - Wait for the app to restore, then open it from your Home Screen.
Also note that apps can be removed from the Home Screen without being deleted. If that is what has happened, the icon in step 3 above will say "Open" rather than displaying a cloud with down arrow. In that case, go to the App Library (swipe left past your last Home Screen). From there you can touch and hold an app and slide it to the Home Screen.