Motley Fool:

Apple is also the only major smartphone manufacturer left that uses standalone modems. Other smartphone vendors use either integrated applications processor and baseband solutions from third parties (e.g., Qualcomm) or make their own integrated parts (e.g., Samsung and Huawei).

And:

On Apple’s job board, the company says it’s looking for a “Sr. Digital IC (PHY) Design Engineer.” Under the “job summary” section of the listing, Apple says the individual chosen for this position “will be part of a silicon design group responsible for digital baseband logic design in state-of-the-art wireless ICs.”

That’s not the only related listing on the company’s job board. Apple also has multiple positions related to RFIC design and layout. RFIC stands for “radio frequency integrated circuit.”

In another listing — this time for a “Sr. RFIC Design Engineer” — Apple says whoever fills the position “will be at the center of a wireless SoC [system-on-a-chip] design group.”

Tricky business, but follows the pattern of Apple trying to own as much of the stack as possible.