Apple historically can’t or won’t bring mixed signal design in-house. It could be related to patents, standards compliance, or the much more complicated silicon design, DV, and test for analog vs digital. It’s not just the modem either. The bulk of Cirrus Logic’s business nowadays is making audio, haptics, and focus servo chips for Apple, and if Apple ever went vertical with those components Cirrus would collapse overnight. Yet Apple has been buying custom audio chips from them for over 30 years, so obviously this isn’t a new trend.