Yeast consumes the glucose and fructose in grape must and produces ethanol, carbon dioxide and heat. As a rough working figure, about 17 grams of sugar per litre yields one percent of alcohol by volume, which is why the ripeness at harvest largely fixes the strength of the finished wine.
Ambient yeast or a chosen strain?
Ambient fermentation relies on yeasts already present on the fruit and in the cellar; inoculated fermentation uses a selected strain. The trade-off is genuine — a chosen strain is predictable and finishes reliably, an ambient population is more variable and is argued to give more complexity, at the cost of a slower and occasionally stuck fermentation.
When it stops
Either when the sugar runs out, giving a dry wine, or when it is stopped deliberately — by chilling, by filtering the yeast out, or by adding spirit as in fortification — which leaves residual sugar behind. Sweetness in a finished wine is almost always a decision about when to stop.
The second one, which is not a fermentation
Malolactic conversion is a bacterial change of sharp malic acid into softer lactic acid. It lowers perceived acidity and produces diacetyl, the source of the buttery character in some Chardonnay. It is standard for most reds and optional for whites, and despite the common name it is not a yeast fermentation at all.