ARM is a proprietary IP and chip design firm. They don't really have anything to do with this stage, but they are proprietary and are set up to support others that are proprietary as well. Like you can get assembly language documentation for the base ARM architecture, but you still won't know all the exact implementation details and peripheral device blocks on the die.

Google took open source software like Linux, prepared it so that manufacturers could add their hardware modules (drivers) at the last possible minute as binaries only. This is called an orphan kernel. While the majority of software on the device is open source, none of the source code for these kernel modules is open source. This is the depreciation mechanism used to steal ownership. No one can ever update that orphan kernel without the source code for the specific kernel modules to run the device. Sometimes you'll find a device supported by custom ROMs long after the device is depreciated. Generally this means someone is doing an enormous task of trying to back port changes and security patches from the present all the way back to the state of the old kernel at the time the last binaries were compiled with the kernel.

The alternative is to merge the source code with the kernel. Once this is done, the community is likely to maintain the kernel modules for a very long time, like decades. Every phone is a little bit different, so reverse engineering one does nothing for the next.

There is more to it still. From the flip side, chip fabs are the most expensive commercial human endeavor in history. They require an enormous up front investment and your devices largely fund the endeavor. This is a major part of the world economic growth. Like the USA was a military spending driven economy until the 1960's. The reason large scale conflict largely ended for the USA has been because of the shift to venture capital and that shift happened in the 1960's because of silicon valley.

So it is a balance between economic growth and the fundamental human right of ownership along with your awareness and expectations in this area. If you do not recognize that you've lost ownership over your property or care, the concept of democracy weakens substantially. You've lost autonomy and that can feel wrong.