Yeast Is Doing All the Work
You do not make beer or wine. Yeast does. Your whole job is to set the table and then get out of its way.
It is humbling to admit, but the brewer does not make the beer. The yeast makes the beer. Everything you do before pitching is preparation, and everything you do after is mostly patience. The single-celled fungus Saccharomyces cerevisiae does the part that actually turns sugar water into alcohol. Knowing what it wants is most of the craft.
What fermentation actually is
In the absence of enough oxygen, yeast feeds on simple sugars and converts them into roughly equal parts ethanol and carbon dioxide, releasing a little energy for itself in the process. That is fermentation, stripped to its core. The carbon dioxide is what bubbles through your airlock and, later, what carbonates the bottle. The ethanol is the point.
But the yeast produces far more than alcohol and gas. Along the way it throws off esters, which can smell of banana or pear, and phenols, which can smell of clove or pepper, and a long list of other compounds in trace amounts. Those byproducts are most of what distinguishes a clean lager from a fruity Belgian ale, and the yeast strain plus the fermentation temperature decide which ones you get.
Temperature is the dial you actually control
Pitch the same wort with the same yeast at two different temperatures and you get two different beers. Warmer fermentation makes yeast more active and more expressive, throwing off more of those esters and phenols. Push it too warm and you get harsh solvent notes (fusel alcohols) that taste hot and rough. Ferment cooler and the yeast works slower and cleaner, which is the whole principle behind lagers.
This is why experienced brewers obsess over holding a steady fermentation temperature and worry far less about the exact temperature of the boil. The boil is chemistry you control directly. The fermentation is biology you can only set conditions for.
Attenuation: how far it goes
Yeast cannot eat every sugar in the wort. The fraction it does consume is called attenuation, and different strains attenuate to different degrees. A high-attenuating strain leaves a drier, thinner beer because it ate more of the sugar. A low-attenuating strain leaves more residual sugar and a fuller body. This is partly why two beers from the same recipe can finish at different final gravities: not because one fermentation failed, but because the strains have different appetites.
Set the table, then wait
So the job comes down to a few things you can actually do. Give the yeast clean, sugar-rich wort. Pitch enough healthy cells. Hold a steady temperature in the range that strain likes. Then leave it alone, because opening the fermenter to check on it mostly just risks contamination. The yeast knows what it is doing. It has been doing this for a very long time, with or without our help. The least we can do is not get in the way.