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it's used to refer to the values of an attribute which are already built-in on Android and available to use such as text appearance, font size, themes, and so on, on the other hand ?attr is used to refer the value of an attribute which you have defined on your own in your application.
you can check it out the link below for further reading: https://developer.android.com/guide/topics/resources/accessing-resources.html#ReferencesToThemeAttributes