How do mobile phones make SMS ringtones with their own music?

First, select the song or music you want to use as a ringtone, and then select Send. At this time, your mobile phone will prompt you whether it is an incoming call ringtone, a short message ringtone or an alarm ringtone. Just choose it directly. Then send the song or music you choose to the ringtone library. Some mobile phones can only be used in the ringtone library. If this doesn't work, then your mobile phone doesn't support it, and you can't help it.