I cant reccomend anything apple or android based, but the PinePhone has some reasonable low-level documentation on itś SoC (Allwinner A64). And there are a great series of tutorials of how Apache Nuttx was ported to the PP. Here's one of the earlier ones: https://lupyuen.org/articles/uboot
I've been working through them myself but still dont understand the boot process, and a long way from radio chip functionality. But I suspect you'll find radio chips quite locked down, since they are licensed for connection to mobile networks. Often there is just a unknown binary blob running the show.
My introduction to this subject was through Harald Welte's work on OpenMoko (i still have an old one lying around somewhere) but after the explosion of iPhones/Android phones have not kept apace. Here is Harald's old but still informative presentation Anatomy of smartphone hardware: Dissecting contemporary cellphone hardware - https://media.ccc.de/v/25c3-3008-en-anatomy_of_smartphone_ha...
Also you might find Roger Ye's old book Embedded Programming with Android: Bringing Up an Android System from Scratch pretty useful.