The decisions, in the order they are made
Wither, to soften the leaf and start its chemistry. Damage it or not — bruise the edges for oolong, rupture it thoroughly for black, leave it alone for white. Fix it with heat, early for green, late for black, never for white. Yellow adds a smothering step after fixation. Dark tea adds microbes after everything else. Then shape, dry, and optionally roast or age. Every category is a different path through the same handful of choices.
What each decision does to the cup
Preventing oxidation keeps fresh, vegetal and marine character. Completing it produces malt, dried fruit and cocoa. Halting it partway generates the floral and stone-fruit aromatics unique to oolong. Smothering removes green sharpness and leaves grain sweetness. Microbial fermentation produces earth, wood and a thick smoothness. Roasting adds toast and caramel on top of any of them. Ageing slowly converts bitterness into depth.
The same leaf, genuinely
This is worth stating because it is hard to believe. A bush that made green tea last week could make black tea this week if the factory decided to. Yunnan’s large-leaf material becomes sheng pu-erh, shou pu-erh, Dian Hong black tea or Moonlight White depending only on what is done to it. Fuding’s Da Bai cultivar makes Silver Needle white tea and Bailin Gongfu black tea. Processing is not a modifier applied to a type — processing IS the type.
Which explains more, processing or origin?
Usually processing, and by a wide margin. A CTC Assam and an orthodox Assam differ more than an orthodox Assam and an orthodox Ceylon. A steamed Chinese green tastes closer to sencha than to Longjing from the next province. Origin matters — cultivar, elevation and climate are real — but if you are trying to predict what something will taste like from a label, read the processing first.