That's because iOS apps are not allowed to use the microphone and camera in background, unless you are actively engaged in something like a call or video/audio

That's because iOS apps are not allowed to use the microphone and camera in background, unless you are actively engaged in something like a call or video/audio recording. Whenever an app is using the mic you will see it clearly in the status bar, it's not possible to hide it. As soon as you close the app completely (swipe up on it in app switcher), those permissions are disabled until the app is launched again by the user. Based on targeted ads I get, it's pretty clear to me that many (most? all?) apps to I grant microphone access to collect and sell data about me. That data is most likely collected from text, behavior inside apps/websites or from recorded audio/video calls. It can even be collected from outside methods like credit card purchase data, emails etc. As a macOS and iOS developer I can say for certain that I have no way to create an app that can listen to you or record video of you while the app is not running. There's simply no API for that.