The hard part is not the modem design per se.
The hard part is designing a good modem while also unambiguously working around all the Qualcomm patents in all the jurisdictions that have iPhone, which is all of them.
Because if you don’t do that, you’re still paying Qualcomm which defeats an important purpose of making your own modem.
Apple is most probably paying Qualcomm for these technologies. They think however that it is commercially beneficial to have their own implementation.
In 2025 it is time to give credit for Apple's PR spreading whole misinformation online during and after the trial. Macrumours and 9to5mac loves to repeat those narrative about Apple no longer has to pay any patent fees when they make their own modem.
As the reply below, Apple still has to pay its SEP. Given Apple has its current deal with Qualcomm until 2027 with time to extend further we likely won't know the full details. Previously it was 5% of Wholesale price for all Qualcomm patents whether they are SEP, wireless or not. With a cap or maximum $20 per smartphone meaning the Pro range don't have to pay a lot more. And rebate towards the modem Apple purchase. The reality after deducting rebate Apple was paying closer to 5%. For reference Ericsson ask for 3% on 5G SEP, previous Cory ruling suggest reality was closer to 2%.
All those patents are standards-essential, which means Qualcomm (and anyone else involved in the standardization process for cellular networks) has to license them under "FRAND[0]" terms, which at least for this scenario means "for the same price we'd license them to Samsung or Intel[1]". It also means that design-arounds are not possible; if you design a different technology from the one Qualcomm owns, you're not speaking 5G anymore. That's why we have these legal rules on standards-essential patents.
[0] Fair, reasonable, and non-discriminatory. Each one of those words has a funny legal definition separate from whatever English you're thinking of.
[1] Who, incidentally, sold Apple the modem division that made the C1, because Intel is nothing but a bottomless pit of bad management decisions