How does Apple know that the other party has seen it by texting?

Apple's mobile phone sends a text message to know that the other party has seen the following:

Tools/materials: iPhone 13, IOS 15.4, local settings.

1, click the information function.

After entering the phone settings, click the information function.

2. Find the function of sending a read receipt.

On the information page, find the function of sending a read receipt.

3. Click to open the function switch.

Select click to open the function switch of sending read receipt.