A baseband is a really fancy specialized SDR. Most are based on arcane VLIW DSP architectures like Qualcomm’s Hexagon or Samsung Marconi - you’ll usually have several DSPs handling the different physical layer channels, and then some coordinating DSPs doing L1 channel mixing and timing (in 4G and 5G, various logical and transport channels are muxed into the same physical channels).
Then a set of higher level processor cores (usually referred to as CP, sometimes still a DSP but often a general purpose application processor like ARM) will handle the MAC and above. There will be occasional fixed function blocks for some common protocol functions, but generally it’s less “analog magic” in the way people think when they hear “radio” and more “DSP magic.”
Seems like the difficult part is doing that effectively while avoiding IP issues -- patents on software and math have entrenched Qualcomm's dominance.
Imagine if all of the IP for ML or AI were held by a single company that got the regulating body (ITU/3GPP) to require their use. Makes a mockery of FRAND terms.
Building a cellular modem, complete with working protocol stack, entirely in software has been done as a 1 person open source project.
Making a production -ready modem is clearly more complex, but far from hard for a company with the resources of Apple.
However, doing so whilst not violating any of qualcomms huge array of patents is the real challenge.
I'm willing to bet that the release date of this phone probably closely aligns with the expiry date of a patent they couldn't work around.