I cannot find how to reinstall a built-in app (App Store) that I accidentally deleted (as I dozed off).

you can't delete the App Store. You must have 'Removed' it

From your Home Screen, swipe left until you get to the App Library.

Swipe downward and you'll get an alphabetical list of your apps.

Locate the App Store and tap/hold until it raises/selects, then drag it and it'll pop out to your Home Screen

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

The App Store cannot be deleted. Go to the App Library after the last Home page and find the App Store icon and you can drag it back to a home screen.