how to restore a deleted app

Download it again in the App Store app. Make sure you are signed into your Apple ID first.

Launch the App Store app, and tap on your account icon in the upper right. Then tap on Purchased. In the next screen, tap on Not on this iPhone. Search for the app and tap on the download icon to download it again.

The app may be getting downloaded to the App Library which you can get to by swiping left until you get past the last Home Screen. You can search for the app in there and when you find it, drag it to the Home Screen.

You can change where apps get downloaded in the settings. See this for more help.

Organize the Home Screen and App Library on your iPhone - Apple Support

The app is probably not "deleted". It is "hidden".

When you get to the App Library screen mentioned by Demo,, you will need to type in the name of the app in the search bar. Once the app appears, you can tap to hold the app until it stills to wiggle, then drag it to one of the other screens.

By re-downloading it from AppStore.

You can just go to the App Store and download it again