Why don't some apps have their own icons on home screen?

I found that some apps in app list of 'Settings - Apps' don't actually have their own icons on home screen.

I wonder whether these apps have some special settings in their 'Application Permissions' or somewhere else. I want to know how different these apps are from the other general apps?

Would you like to help me to make a technical explanation for these special apps?

Thank you in advance.

Some are System apps...are those the ones you mean?