I've started to write upe some information on modern smartphone hardware architecture. It will be in a similar style to what I previously wrote on feature phones and gsm modem hardware, but with a specific focus on smpartphones, their multiple processors, memory sharing, AP/BP interface, audio architecture, etc.
I should have done this a long time ago. In fact, I think I should write more documents like that on various technical subjects. If you want to learn about low-level aspects of modern telephones, there is way too little published information out there.