I am certainly no Apple engineer, nor phone OS coder, so I can't go TOO deeply into this hole. You may need to consult with Apple as to WHY this is what it is. The thing is that AAC, AptX, etc... are not the Bluetooth driver/stack. These are just the codecs, which is what the OS tells the BT stack (Flouride on android) to encode in order to be transmitted and then decoded on the IEM side of things. You are right this has nothing to do with the volume information (except if the song file itself contains gain levels/volume leveling numbers).AAC, or for that matter, AptX, SBC et al, are a transmission mechanism that code then decode a file to be moved from source to receiver in as small a volume as its parameters allow. Nowhere in the Bluetooth standard is there reference to volume control. Apple still have to comply with those standards at least.
And as for the Bluetooth standard, this is also not specific to any one OS, but coding can be inserted before or after the stack (or if they really want to right in the stack). Windows uses many many different Bluetooth stacks (some much better than others). Android only tends to use one at a time. And, to be quite frank, Apple doesn't always adhere 100% to standards because it is such a closed system. They might have to comply to standards that are not their own pieces of software/hardware, but if it is written specifically for IOS and only IOS they they need not comply with anything that is not patented. In the case of Bluetooth they only need to comply with the transport protocols that allow connections, signal transmissions, etc.. There is nothing saying that they cannot add volume leveling in the code if they so choose.
This, actually is exactly what I am telling you. There is indeed intervention from an app going on here. Think of the system volume on IOS as an app, just like Neutron, or etc... Except it overrides all other apps asking for volume information. It IS and does control the volume level of transmitted signals over all outputs possible rather than leave it alone as a "transport" only situation. This is also why your Sony limited headphone jack but not the BT volume, because they chose to leave it that way.and does not explain fundamental low volume over Bluetooth without app intervention.
What I am arguing here is not whether this phone limits volume or that one doesn't, I was simply trying to say that MOST android phones (and especially plain android DAPS) do not limit volume in any way concerning Bluetooth.
Which is exactly what I was trying to explain after you said:but honestly that just shows me you can control volume through certain applications
If the iPhone has no control or knowledge of the Bluetooth volume, how can it limit it?
The bottom line is: IOS Bluetooth max volume is lower than on most android devices but not because of the power differences on the device. It is purely on a software level.