Winemakers can make either a dry wine or sweet wine.
If the winemakers want to make a sweet wine, they must artificially halt the fermentation process by either increasing or decreasing the temperature in the fermentation tank to a certain level in which the yeasts cannot survive. Manipulating the temperature kills the yeast (well, actually they lay dormant) and it allows natural sugar to be left in the wine. The high sugar concentration of the grape plays a role, too.
There is a second way to artificially halt the fermentation process, it is called fortification. Winemakers will add alcohol during the fermentation process to a level in which the yeasts cannot survive, also leaving natural sugar in the wine.
And by the way, Moscato is purposefully made in a sweet style which is why it is always sweet.
I hope this helps.
Moscato is a sparkling white wine made in the Piedmont region of Italy. It is made with Muscat (Moscato) grapes. They are fermented naturally until they hit 5.5% ABV and then the wine is run through a sterile filter to preserve the sweetness and stop fermentation. It's then artificially carbonated and put in a bottle. It's meant to be an dessert wine because of this sweetness. Also known as Moscato d'Asti and there is a similar wine made nearby simply called Asti.