The grapes are harvested at desired level of ripeness (brix).
Then the grapes are crushed and , the juice is being separated from the skins.
The juice is left to ferment only partially. When the desired level of alcohol is reached, the wine is chilled and filtered, leaving some un-fermented sugar in the future wine.
This is the reason why Moscato d'Asti are low in alcohol and fairly sweet. Not all the sugar has been transformed into alcohol during fermentation.