Few natural ferments achieve 100% conversion of sugar molecules to alcohol because the yeast would die in such an adverse environment. Distillation separates out 15-16% alcohol by heating above its boiling point (typically around 79.4 Celsius or 175 Fahrenheit) vaporizing and then cooling via condensation into higher proof distilled products.
A still is a large tank with both an evaporation section and cooling condenser for cooling the vapors that have passed through an evaporation section, before passing them through a cooling condenser, where their concentration of water and alcohol (neutral spirit) is further reduced by passing them through this component. Distillers also remove any poisonous methyl alcohols found at the start of vapor production which could potentially cause blindness if inhaled; remaining neutral vapor known as heart cut is removed before passing them through condensers to reduce concentration to neutral spirit or neutral spirit concentration levels.
More refluxing (i.e. repeating of this process in a column), means more concentrated distillate will result. Distillers use different amounts of refluxing for different flavors.
While distillers’ goal is to craft high proof products without diluting, distillers also separate out various congeners (the non-alcohol byproducts of fermentation) that give each spirit its unique character and create its flavor profile. Distillers may keep some congeners while discarding others to tailor each spirit batch according to its profile; this process doesn’t lessen its taste but simply allows for greater control over its identity.