Open the Shortcuts app
Tap the plus icon
Tap Add Action
Search for and select Open App
Tap App and choose the app you want to customize
Tap the down arrow at the top
Tap Add to Home Screen
Tap the icon next to New Shortcut
Choose Take Photo, Choose Photo, or Choose File
Select your custom icon image
Enter a name for the shortcut
Tap Add
Repeat for each app icon you want to change
Optionally hide the original app from the Home Screen by removing it from the Home Screen, not deleting it