Is it free for Apple phones to send messages to each other?

It's not a text message. There's no charge. It's iMessage You need to set-information, turn on iMessage, and then you can send messages to each other in cellular data or WiFi environment. The criterion is that the send button turns blue, indicating iMessage.

Method steps for sending free iMessage:

First, find the settings on our mobile phone desktop, and click, as shown in the figure below.

Then click Information, as shown in the figure below.

Then click to open the iMessage information switch, as shown in the figure below.

Then return to the main interface of the mobile phone and click Info, as shown in the figure below.

Then send a message to your friends who have enabled this function.