My Find My app icon has disappeared and I don’t know how to get it back. The app is turned on but how do I get the icon back

It is all merged into a single Find My app that should already be installed when you upgraded to iOS13.

Look for it in the Extras, or Utilities folder.

Read this link please:

Apple - Find My iPhone, iPad, Mac, and Apple Watch - http://www.apple.com/icloud/find-my-iphone.html

"The new [iOS13] Find My app combines Find My iPhone and Find My Friends into a single, easy-to-use app on iOS, iPadOS, and now macOS.* Find My can help you locate your devices and your friends and family. And now it can help you find your devices — even when they’re offline."

It cannot be deleted. Swipe down on the Home screen and search for it.

Not a big green, radar screen thing? Are you sure you didn't hide it in Screen Time?

Restart your phone. See if that helps.