Miniaturized, high frequency, low-power, high bandwidth, radio modems to be exact - supporting multiple complex protocols. In real-time. Meaning it is all done in hardware, although firmware does some management stuff. It's about as hard as silicon engineering gets.
Then, the protocol stacks on top of that are enormous and complex. The software is very large.
Broadcom also tried and failed because they underestimated the task, IMO.
Those who can/could make it happen are those who are willing to invest massively over many years and, ideally who have a sure customer. E.g. Huawei who is willing to take the long view and who ships hundreds of millions of phones.
( Disclaimer: I used to work there, could give more details, but NDA )
then they can‘t say anything because of nda. or the gsm standard manual costs hundreds of dollars.
my tin foil hat is glowing, i‘m telling you. we already know that gsm encryption is a joke. i don‘t want to imagine what‘s going on inside all those closed-source baseband firmwares. it creeps me out.