I doubt that if the original file is AAC, it gets decompressed and recompressed in the source device. I guess that if the original file is AAC you’re in good luck and you’ll have a better intact AAC file transmitted without recompression. Maybe I’m wrong.

It doesn’t matter whether the playing audio file is in AAC format or not. What we’re arguing here is AAC transmitting bluetooth codec not an audio file encoded and decoded in AAC format. AAC bluetooth codec is different than AAC audio files. Whatever file type you play on iPhones whether MP3, AAC, ALAC, MQA,… will eventually be transmitted through AAC bluetooth codec/ Encoded and Decoded. I hope this clarifies what you’re trying to figure out.