Open the App Store app on your iPhone or iPad
Tap your profile icon in the top right
Tap Purchased
Tap Not on this iPhone or Not on this iPad
Find the app you want to redownload
Tap the download icon next to the app
If the App Store app is missing, open Safari and go to apple.com/app-store
If you mean the App Store itself is missing, check for restrictions in Screen Time
Go to Settings
Tap Screen Time
Tap Content & Privacy Restrictions
Tap iTunes & App Store Purchases
Make sure installing apps is allowed
If the App Store is still missing, restart your device
If needed, update your iPhone or iPad to the latest iOS or iPadOS version