Is it really all one plant?
Green, white, oolong, black, yellow and dark tea look nothing alike, taste nothing alike and are shelved as different products. All of them come off Camellia sinensis. If the plant is the same, where does the difference actually come from?
Short answer
From what is done to the leaf after it is picked. Every type is the same leaf sent down a different route, and two decisions carry most of it: how far oxidation is allowed to run, and when heat arrives to halt it, if it arrives at all. A leaf that meets the pan within hours is green tea. The same leaf withered, broken open and left to oxidise fully is black tea. Stopped partway, it is oolong. Cultivar, place and season are real, but they set the material rather than the category: one morning's pluck off one bush can leave the factory as a green tea or a black one.