How does Apple switch between primary and secondary cards when sending text messages?

Tool: iPhone 8

Operating system: iOS 14.4. 1.

Program: Information 2.0

1, open your mobile home page, and click Info to enter, as shown in the figure below.

2. Enter the information interface and click New, as shown in the figure below.

3. Enter the new interface and click the recipient, as shown in the figure below.

4. Enter the recipient number or add an address book, as shown in the figure below.

5. After adding the recipient, the default mobile phone card name will appear. Click on the mobile phone card, as shown in the figure below.

6. The option of two cards appears, and switch to the card you want to use, as shown in the figure below.

7. If the handover is successful, enter a short message and click Send, as shown in the figure below.