The peaty flavour in whisky comes from a compound called phenol which comes from the Peat when it's burned to dry the barley after the malting process. Through fermentation, distillation and ageing, the phenols stick around and give whisky that lovely smokey flavour.
Spicy & Woody
The spicy and woody flavour in whisky comes from the cask, quercus alba (white oak) and European oak - Vanilla spice and all things nice. Over time, the flavour becomes more prevalent and complex in the whisky.
Fruity spirits we love from sweet to citrusy we love it all. In whisky when distilleries have these long fermentation times they produce esters, these are compounds that can give whisky that fruity flavour. Longer the fermentation, the more esters, the more fruit compounds that are packed into the whisky. Over time these evolve into long complex flavour profiles that we love to explore
If the cask has previously filled with sherry, wine or port it leaves behind these lovely big bold tannins, rich red fruits and long finishes. With these maturations the cask absorbs all those lovely flavours so as to when whisky fills that cask it gets to pull all those lovely flavour compounds out.
The cerealy world of whisky all comes from those many strains of barley you have in Scotland. Distilleries spend decades perfecting their strains of barley and how to use them. When you bake a cake it all starts with the ingredients; great ingredients produce great cakes. Whisky is the same, the better the barley, the more crafted that barley is for the distillation the better the whisky is. When its good it really shines through giving us that malty, husky, cooked vegetable notes that we love.
Aldehydic compounds were championed by the perfume industry, it gives perfume those lovely leafy, grassy and floral notes. We find these in whisky today, through a combination of fermentation and distillation it can produce these lovely Aldehydic compounds giving whisky that lovely fragrant and greenhousey flavour