This follows from their multi-year agreement.
As you know, last year Apple and Qualcomm settled a very long dispute and agreed on a new stage of cooperation.
As part of this process, Apple will again use Qualcomm modems in its smartphones. In particular, the upcoming generation of the iPhone 12 should get a Snapdragon X55 5G modem.
Various rumors indicate that Apple may bring to market the first devices with its own modem as early as 2021. However, as it turned out, the agreement between Apple and Qualcomm indirectly hints that, if this happens, Qualcomm’s modems will not stop using the Cupertian giant for long enough.
The agreement implies that Apple will use SDX55, SDX60, SDX65, and SDX70 modems. Even if you consider that Qualcomm will release one new modem per year, it turns out that Apple will use them at least until 2023 inclusive.