How does imessage set the mobile phone number?

Taking iPhone 1 1 as an example, the method of setting imessage as the mobile phone number is as follows:

1, open the desktop settings, as shown in the figure below.

2. Click on the "information", as shown in the figure below.

3. Click "send and receive", as shown in the figure below.

4. Finally, choose the mobile phone number as the sending method, as shown in the figure below.