I deleted the App Store, how to download it again?

The App Store app is not one of the apps that can be deleted but it can be removed from the home screen. 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.

Another reason for it not being visible is due to a restriction on purchasing. Go to Settings and tap Screen Time, tap Content & Privacy Restrictions, tap iTunes & App Store Purchases, choose a setting and set to Allow. See: Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support