Most BT inputs take the volume off the phone as the primary, then your volume nob will be secondary.
So I turn my phone to full 100% then you the cars valour knob to alter the volume.
With any aux input, you will see volume drop off, some phones it can be as much as 50%, but mostly a 20-30% hit.
This has never bothered me, as I listen to books and don’t need my sound to be very loud.
A the anker device John is just a dumb through. Set your input device to full and your radio volume knob/button will control your car speakers.