How does iphone send short messages with secondary numbers?

Operating system: iOS 14.4. 1.

1. First click the information on the iPhone interface, as shown in the figure below;

2. Click the Create button in the upper right corner, as shown in the figure below;

3. Click the "Main Number" button, as shown in the figure below;

4. Select the secondary number from the following dialog box, as shown in the figure below;

5. After editing, click Send to send a message with the secondary card, as shown in the figure below;