What does changing the volume on a Bluetooth audio device really do?
So, there are two things at work here:


The digitally encoded audio that the phone sends can just be scaled, leading to a louder (or less loud) reproduction by your receiving end
Bluetooth's audio profiles do indeed specify commands to change the reproduced volume.


That's not "metadata" (that would imply synchronization with a specific point of audio); it's commands that are simply not part of the audio flow. Bluetooth is not a "single purpose link"; it can encapsulate multiple types of data.