Apps like Bluetooth Mono Router & BTmono let you hear music on old Bluetooth (BT) earphones that were mono (meant for voice calls only since they don’t support A2DP profile) .
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 to input audio into a video recording. 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
This can’t be done with the ear sets supporting latest BT 5.0.
How does this work?. (I’ve spent several hours unsuccessfully searching the net and sister SE sites) .