Bluetooth audio as input for video recording Apps like Bluetooth Mono Router &amp; BTmono let you hear music on old Bluetooth (BT) earphones that were mono (meant for voice calls only since they support Headset Profile (HSP) and don't support A2DP profile) . (Edit : MonoBt play is another such app)
So, if you have such mono sets lying around, you can use them for listening to music, YouTube, etc which is good.
There is a more interesting aspect to it - these allow you to use your BT ear sets  (whether mono or the latest ones supporting BT 5.0 ) to input audio into a video recording on your cell phone. You can comfortably record videos with your BT audio input as long as you are in range, without using wired microphone or UHF transmission. See Bluetooth input for video recording : Android 10, One Plus 7
How does this work?. (I've spent several hours unsuccessfully searching the net and sister SE sites) .
I requested developer Bluetooth Mono Router to explain this and if possible answer the question here but there is no response. What I could gather from comments and reviews is this :


App creates a virtual call environment thereby letting audio (music etc) be treated as voice and hence you can hear using mono ear sets meant for voice calls.
Recording of audio is not  by design. It's an incidental artifact of the virtual call feature. It's not a design feature.
This has nothing to do with Bluetooth versions