Wines get their flavor and aromas from a combination of the grape varietal used, byproducts of the fermentation process controlled by the winemaker, and the aging process including whether or not barrels are used, the type of barrels, and how long the wine spends in the barrel.
When you put the wine in your mouth and you pull air in at the same time, what that is doing is pulling all those flavors into your mouth and actually up into your olfactory bulb. It pulls all the flavors up into your nose, because when you are tasting you are smelling at the same time.
Wine is buttery from going through a second fermentation that converts bracing, stringent acidity in malic acid to a softer, creamier lactic acid while also producing diacetyl which is the same compound used for fake movie popcorn buttery aroma.
"Nice Legs" in wine refers to how thick and slow or thin and fast the wine tears down the side of the glass after swirling it. Thicker slower forming "legs" means higher alcohol while thin like water means lower alcohol. It is not good or bad, but it sounds good to say "nice legs".