How does Apple switch between blue and green when texting?

Apple SMS blue-green switch is as follows:

Tool: iPhone 8

Operating system: iOS 14.4. 1.

Program: SMS

1. Open the phone interface and find the SMS portal.

2. Open the message and you can see all the messages.

3. What others reply to you is a gray box, and what you reply to others is blue and green.

4. Blue indicates that you are both connected to the Internet, both using Apple phones, and the sending method is imessage.

5. Green means that some of you are not using Apple phones, or some of you are not connected to the Internet. In this case, send an ordinary short message through China Mobile.

6. The only difference between the two is that there is no charge for the blue one, and so is the green one. If there is no SMS in your package, charge 0. 1 yuan/article.