Here modem == baseband. The SIM is separate to the baseband. SIM cards run their own OS that can run software from carriers, look at osmocom.org research for mo

Here modem == baseband. The SIM is separate to the baseband. SIM cards run their own OS that can run software from carriers, look at osmocom.org research for more. An eSIM is still a physical device, just soldered onto the motherboard, but still separate to the baseband. The baseband is always software running on hardware, its just not yours to control, and very complex to reimplement even if you had control. Apple only just managed to replace Qualcomm basebands with their own version recently, and they had to make their own baseband processors to be able to do that.