How does Apple mobile phone switch between primary and secondary cards to send messages?

1. First unlock iPhone 1 1, open the information in ios 13.5, and click on the upper right corner to prepare the information.

2. Then, in the editing page that opens, enter the phone number of the recipient, as shown in the figure below.

3. At this point, a green button will appear below, indicating that the main number is in use. Click the button, as shown in the figure below.

4. Then, in the page that opens, select the secondary number, as shown in the figure below.

5. Finally, write the information content, click the green arrow to send, and complete the primary and secondary number switching.