> 3) If a background app starts then stops recording audio while the screen is off, would you have an indicator that it recorded audio?

Yes. iOS displays an indicator if an app has recently used the mic.

> Note: Whenever an app uses the camera (including when the camera and microphone are used together), a green indicator appears. An orange indicator appears at the top of the screen whenever an app uses the microphone without the camera. Also, a message appears at the top of Control Center to inform you when an app has recently used either.

Source: https://support.apple.com/en-nz/guide/iphone/iph168c4bbd5/io...