Having the latest BigSur there is this strange behaviour:
Built-in Input becomes -> Headphones only after restart, and then only playback is possible, mic is not working. I am using combo 3,5 jack, not the one on the dock.
When using OBS broadcast software, I cannot capture the playback sound in the recording, only mic is possible.
I tried the iShow audio capture, but still cannot see the iShow interface in the list to aggregate the output to that, and have this captured in OBS.