When setting up an A/B Test, you will see the option to enable/disable this toggle:
If this toggle is NOT enabled, then when the schedule is completed, the A/B Test will continue running.
To stop the A/B Test you will need to complete it.
In this case, the A/B Test will be deleted from the Composer canvas and only the winning variant's branch will remain in the Experience.
The winning variant is calculated based on the KPI selected. It might happen that there are 0 conversions for both variants, however, the winner is the one with the lower number of executions.