There are many reasons for Apple's self-developed baseband, one of which is to realize a comprehensive self-developed processor and communication baseband. In addition, Apple faces the global market, and iPhone needs to be compatible with more frequency bands in different regions, so it needs to support more 5G frequency bands and TD-LTE and FDD-LTE frequency bands.
However, Apple's accumulation in related technologies is relatively weak. For example, the number of necessary patents for the 5G standard is not ranked high in the global 10, which is quite different from Huawei and Qualcomm. Therefore, Apple's self-developed baseband may not necessarily make the iPhone signal better.