Why does this English thing always appear when I send text messages on my Apple phone? If you change the object, a normal message sending interface will appear.

This is Apple's own iMessage service. Only when both parties are Apple mobile phone users, and the iMessage service is enabled and there is a network environment, the SMS will automatically switch to iMessage status.

When you say that the other party doesn't have iMessage, it's very likely that the other party is not an Apple mobile phone user, or the network environment of you or the other party doesn't support it.