How does Apple's mobile phone set the information that the other party has seen?

You can find this problem by trying two Apple phones.

Under normal circumstances, Apple has the function of receiving short messages.

This function requires the other party's Apple device, and the other party has opened the read receipt.

In Settings-Info-Open Read Receipt. As long as the other party opens it, you can see whether the other party has read it.

identical

If the other party doesn't turn on this function, you can't see it, only see whether the information is delivered to the other party's mobile phone.