How does Apple send messages with id?

Open settings-> Information information, open imessage and log in with apple id. Imessage can only be sent to people who also use imessage (just as WeChat can only be sent to WeChat users).

When sending imessage, you can use the other party's apple id (usually email address) or the bound mobile phone number.

For example, if you open imessage on iphone and other imessage users send messages to your mobile phone number, it is imessage.