how can I locate the app using a microphone (green or orange dot) on my phone

Control center is supposed to tell you what's using the components

Access Control Center on your iPhone and iPod touch - Apple Support

About the orange and green indicators in your iPhone status bar - Apple Support

Most often it is the phone or Siri for the orange dot because both use the microphone. If you open the Control Center it will name the app at the very top of the screen.

yes

No, it will show for a while after the dot goes out.