How do I restore the App Store on my iPhone 7 running iOS 15.8.4?

Unless you are in the EU, it cannot be deleted on your phone. There are 2 reasons why you may not see it:

• It has been removed from your Home Screens and you will find it in your App Library where you can add it back to one of your Home Screens. Swipe all the way to the left to go to the App Library and swipe down to reveal the apps alphabetically. If you see the App Store, move it to a Home Screen.
• If you are part of a Family Plan or using Screen Time Restrictions on your phone, then the App Store can be hidden. Go to Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Installing App, and make sure it is turned on. If you are part of Family Sharing, it is the Organizer that has control of your phone and you will not be able to make the change yourself.