My iPhone plus cannot download find my friends app and should that app be already installed in my iPhone 8 Plus????

Are you using iOS 12 or iOS 13?

Should be the latter:--> In iOS 13 or later version, there is a new app called "Find My", which contains (and replaces) Find My iPhone and Find My Friends apps.

See this page for more info:-->  Find My – Official Apple Support

This new app is already available on your iPhone (if updated to iOS 13 or later). This is the related icon:

You should be able to locate it:

in the same position where Find My iPhone icon was (using iOS 12);

or in Extras folder (or a different folder);

and in Settings>[your name].

Regards

Giulio

With iOS 13 the app was redesigned, renamed to Find My, and is already on the phone. It may be in the Extras, or Utilities folder