I thought I'd find out how they do it properly.
I turned to the Adnams Brewery, and enlisted the help of their Quality Manager, Belinda Jennings, who I first met in a field in Suffolk...
Step 1: Raw materials
Water, malt, hops, yeast. That's it, for a proper beer.
What affects the final flavour is the way these things are treated.
The softness of the water, how dark the malt is roast, the species of hop, the strain of yeast.
Malt adds sweetness, and provides the sugar for fermentation. Hops add bitterness, especially to balance the sweetness of the malt, and the yeast, of course, turns the sugar into alcohol.
Most brewers (Adnams included) will happily give their recipes, but they won't give their yeast - established brewers have strains that are slightly different to other brewers' strains, and so affect the flavours. Adnams have been using the same strain of yeast since 1945.