For privacy reasons I'd like to know when the camera and/or mic are being used by an app. Is there an app or a built-in feature to shows an icon to indicate that?
There’s a product called Oversight that does this. It’s a donation-ware product meaning it’s completely free but donations to support development are encouraged.
It’s accessed through the menu bar, but if something accesses a device, it will post a notification
I’ve used this before for users who who were concerned about malware accessing their camera and it worked well. I haven’t tested in Catalina, but it should work though (IMO) it’s not necessary due to the security permissions model in Catalina. If you don’t grant an app access to a device, it can’t use it.
Micro Snitch by Objective Development: