How does Apple send information by phone number?

How does Apple send short messages with the secondary number?

1. First, open the settings in the home page, then find "Cellular Network" in the settings, and then click it.

2. Then, in the cellular network, we open the settings of two cards and click on two cards respectively.

3. We click on the data flow card, then open this card and click "Enable this number".

4. After the two cards are activated, we enter the SMS page. Then click Data Flow.

5. Click on the data stream and we find that we can change the card we use. We choose the secondary card here, that is to say, we use the secondary card to send text messages. After clicking, we can see that we are using another card.