App[le did not create the AAC CODEC. It was created by the German company Fraunhofer, as a successor to the previous MP3 technology that they had developed. If Apple had developed AAC, then nobody else would be allowed to use it, period, which thankfully is not the case.
A lot of laymen think so but it is not true.
It must be PCM otherwise you can’t do any kind of DSP like volume control, EQ, mixing (incoming phone call).
Finally the audio is send to the audio device, in this case the Bluetooth sender.
There the audio in encoded with the codec negotiated between sender and sink. The sink decodes it to back to PCM and send it to the DAC of the Bluetooth device.
Robert Trigss did an elegant experiment.
He took a AAC file, so one with a known frequency response and played it on a couple of phones including a iPhone.
As the frequency response differs from the original, obvious the AAC file has been decoded and encoded by the Bluetooth device using AAC as the transmission protocol.