Activation code of a widget

Good afternoon:

In the php files of wordpress and related to widgets, there are functions in the code of this type:

if ( is_active_sidebar( 'above-header-right' ) ) :
				dynamic_sidebar( 'above-header-right' );
				endif;

That is, if the widget is active, the function is executed.
I need clarify the following question:
What code do you know if a widget is active or not with? that is, how does the above function (code level) know if a widget is active or not?

Thanks