Open the Shortcuts app
Tap the plus icon to create a new shortcut
Tap Add Action
Search for and select Open App
Tap App and choose the app you want to customize
Tap the down arrow or info button at the top
Tap Add to Home Screen
Tap the app icon next to the name
Choose Take Photo, Choose Photo, or Choose File
Select your custom icon image
Rename the shortcut if needed
Tap Add
Repeat for each app icon you want to change
If needed, move the original app to the App Library or another Home Screen page