[zarq]

The dot seems to not be interactive, i.e. I won't be able to find out which app it was, and what it did. But it's a start!

[judge2020]

Open control center. At the top it'll say what app was using the mic or video.

https://i.judge.sh/frank/Derpy/chrome_0aklMR0vP4.png

https://i.judge.sh/careless/Flutter/chrome_Vm3HAfRnaQ.png

[asadlionpk]

If you drag open the control center it tells you which app used it recently.

[hackmiester]

The dot is just for the foreground app. If a background app uses the mic, your clock turns red (for recording) or green (for a phone call) in an even more obvious way, and tapping it takes you to the app that is doing so.