Earlier quoted context omitted.
> https://imgur.com/a/ifaiT I can see "Messenger" and "Instagram" with access to microphone.
Those apps have voice and video features, so it makes sense that they'd request microphone access. (And iOS lets you disable those specific features if you want.) iOS also shows an indicator if an app is using the microphone. This is true even for built-in Apple apps like Voice Memos. There's no way to surreptitiously record.
The banner won’t show if the app using the mic is in the foreground.