After upgrading my iPhone 8 with IOS 13.1.3 I cannot find nor retrieve any longer my APP: "Find Friends"

iOS 13 combines the Find My iPhone and Find My Friends functionality into a new single app with the name "Find My." It is installed on your phone as part of the upgrade and is in the same folder where you previously had Find My iPhone. You can use Search to find it if needed. More information:

Share locations in Find My on your iPhone and other devices - Apple Support

Set up Find My on your iPhone, Mac, and other devices - Apple Support

You cannot install the Find My Friends app when running iOS 13. You need to use the "Find My" app; it is on your phone but may be hard to locate. See the following from the link in my previous reply -- it will help you locate it:

If you need help finding it, use Search on your iPhone, iPad, or iPod touch,