how do I uninstall an app, then reinstall it ?

First disable the Screen Time Restrictions, if any.

Settings > Screen Time > Contenet And Privacy Restrictions > iTunes & App Store Purchases > Deleting Apps > [Allow]

There are six possibilities

1. Delete an offloaded App
2. Delete an App from the Home Screen
3. Remove an App from the Home Screen - Will be placed in the App Library
4. Delete an App from the App Library
5. Delete an inbuilt App - Not feasible
6. Delete an inbuilt App with one feature - You do not get any options either (e.g. Magnifier)

Watch the GIF