A2DP (Advanced Audio Distribution Profile)
The Bluetooth profile that streams stereo audio one way, from phone to headphones, and the one that decides which codec gets used.
Classic Bluetooth is a toolbox; profiles say what to build with it. A2DP, adopted by the Bluetooth SIG in 2003, covers one-way, music-grade audio: the source encodes, the sink decodes, and the two negotiate on connection which codec they share. SBC is mandatory and therefore the floor. AAC, aptX and LDAC are optional and only kick in when both ends support them, which is why the same pair of earbuds can sound different on an iPhone and an Android phone.
There is no microphone in the profile. When a call comes in, the device has to switch to HFP, and the sound drops from stereo to something reminiscent of a 1990s switchboard. The headphones aren't broken; that's the profile change. LE Audio with LC3 is the first serious attempt to get rid of the split.