The avocado sandwich turns on a single decision made before any bread is touched: sliced or mashed. A fanned slab of ripe avocado holds its shape and gives distinct, buttery layers but slides as a unit when the sandwich is bitten, shearing out the side. Mashed and seasoned, it behaves like a spread, gripping the crumb and staying put, but loses the sense of a thing you are eating in favour of a paste. Almost everything that makes this sandwich good or bad is settled by which of those two states the avocado is in, because avocado is not a component here. It is the entire filling.
The craft is compensating for what avocado lacks. The fruit brings fat and a smooth, faintly grassy richness but no salt, no acid, and no contrast, so a sandwich of avocado on bread is one soft, mild note unless it is corrected. Salt is not optional seasoning here, it is structural, and a squeeze of lemon or lime does double duty by sharpening the flavour and slowing the brown that ripe avocado turns within minutes of being cut. The bread is usually given some character it does not have to invent on its own, a toasted slice or a seeded loaf, so there is texture against the softness. Made and held, it greys and weeps, which is why it is a sandwich built and eaten close together rather than packed for hours.
The variations are mostly the addition of the contrast the plain version has to manufacture: chilli flakes for heat, tomato for acid and water, a hard cheese for salt and bite. Each pushes the sandwich toward a named pairing of its own, and those deserve their own articles rather than being crowded in here.