If you find that your iPhone is playing music by itself, there are a few possible explanations. First, it's possible that you have an app on your phone that's set to play music automatically. To check for this, open the Settings app and tap on the Music tab. If you see an option for Playback Options, tap on it and make sure that the Autoplay Music setting is turned off.
Another possibility is that you've accidentally set up a Voice Command shortcut that triggers music playback. To check for this, open the Settings app and tap on the Voice Control tab. If you see an option for Customize Commands, tap on it and make sure that there aren't any commands listed that could trigger music playback.
Finally, it's also possible that your iPhone is playing music because of a bug or software issue. If you think this might be the case, try restarting your iPhone or restoring it from a backup.