Apps cannot use the microphone on iOS 14 without your awareness. There is a little dot that will show up at the top of the screen if an app has recently used microphone or camera. Hopefully this can assuage your paranoia.
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.
