How do I out find iPhone back on my phone

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);
• in Extras folder (or a different folder);
• and in Settings>[your name].

Regards

Giulio