How do I check the active audio codec for a bluetooth connection in Big Sur?
Download Bluetooth Explorer, open it, then navigate to Tools &gt; Audio Graphs. You can see at the bottom the active codec - in this example I'm using AAC.

Note that Bluetooth Explorer is no longer included in the latest Xcode Additional Tools bundle. The last version I could find was for Xcode 11, and it now crashes sometimes on Big Sur, but still does the trick. Download it here: https://developer.apple.com/download/more/?=additional%20tools