What is the red dot up in the right corner of an iPhone

"An orange indicator

means the microphone is being used by an app on your iPhone. This indicator appears as an orange square if the Differentiate Without Color setting is turned on in Settings > Accessibility > Display & Text Size."

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

The red dot appears at the top of your screen whenever an app is using your microphone or camera. When you dial a phone number and then begin talking, then you are using the microphone and therefore the dot.

Axel F.