I didn’t ask how it works. I understand how it works, that’s not the problem.
I am repeating myself, I don’t know if this is gonna help me in some way, but at least I hope you’ll understand our problem (mine and richard’s):
As I already stated in my previous reply, the problem appears when I try to play my music on my iPhone 6s (iOS 11.0.3) by any bluetooth device or even through AUX cable (as I newly found yesterday). With the volume maxed out, it is still quieter than it used to be (before I updated to iOS 11). As soon as I receive a notification on my unmuted iPhone, the music volume on the connected device is getting higher instantly like I was used before. And (as I experienced until now) it stays the same until I change the app that plays the music or I receive a phone call. That’s why I know there is an issue and it wasn’t supposed to work like this.
All this is not about how to change the volume on the device, it’s about how loud can it get.