How does Android send text messages for free?

There is no charge for texting. Just some software, such as Yuni SMS and Fetion, but the other party can't send it to him for free without these software, and it can't send it without traffic (since it's Android, it's connected to the surrounding Wi-fi). In short, you need to download and install some software and make sure that the other party has the same software, ok? I see. Please rate it as a satisfactory answer.