How do apples get sweeter after getting picked from trees?
Fruit ripens due to the effects enzymes present in the fruit degrading certain chemicals like starches into sugars which happen to taste sweeter to us. Other things break down as well, which is why there is a color change. The rate is increased by ethylene gas given off by the fruit, which is why you can increase ripening by putting fruits in a bag so the gas doesn’t dissipate as fast
Fer·men·ta·tion starts the second the apples plucked from the apple tree.
Fermentation is the chemical breakdown of a substance by bacteria, yeasts, or other microorganisms, typically involving effervescence and the giving off of heat.
In the short term this sweetens fruits as their natural sugars breakdown. Overtime this is also why it ramps up the decomposing process.