Missing icons

iPhone

Try this: Use App Library to search Photo app, see this Apple article:

Organize the Home Screen and App Library on your iPhone

Organize the Home Screen and App Library on your iPhone - Apple Support           

If you have find the apps, tap & hold on the app, then drag to the Home screen & drop it.

iPad

Use the App Library and Home Screen to organize your iPad apps

Use the App Library and Home Screen to organize your iPad apps - Apple Support

Find your apps in App Library on iPad

Find your apps in App Library on iPad – Apple Support (UK)

Then see this Apple article: Move apps and create folders on your iPhone, iPad, or iPod touch

Move apps and create folders on your iPhone, iPad, or iPod touch - Apple Support