Lemon curd is the sharp-sweet spread as it comes out of the jar, the lead the made sandwich is built from rather than the sandwich itself. It is a thick set preserve of lemon juice, lemon zest, sugar, butter, and egg yolk, cooked gently until it holds its shape on a spoon, with a flavour that is sour first and sweet second. That order is the defining fact. Most sweet sandwich fillings are sugar with a top note added; lemon curd is acid carried by sugar, and the butter and yolk in it give a density and a glossy slackness that jam does not have. It spreads like a thick custard rather than a jelly, which is what decides how it behaves on bread.
The craft, on the British tea table, is in the ratio and the barrier. The curd is sharp and rich at once, so it is spread thin: a thick layer is cloying and, because the yolk keeps it loose, it slides under any pressure and bleeds at the cut. Soft white bread is buttered to the edges first, and the butter is structural here, not flavour. It waterproofs the crumb so the curd's moisture does not soak straight through, and its salt is the thing that stops the sweetness reading as flat and lets the lemon's sourness come forward. The bread stays plain and soft because the curd brings all the flavour and has no texture of its own, so any crust with real chew would fight a filling with nothing to answer it.
The variations are a matter of what the curd is set against rather than changes to the curd itself. A thin layer of clotted or whipped cream cuts the acid and turns it toward a teatime indulgence. A scrape of a sharper jam alongside doubles the fruit and the tartness. The other made jarred-spread sandwiches, marmalade with its bitter peel, the sweeter berry jams, run the same soft-bread and salted-butter logic on a different preserve. Each of those is its own build and deserves its own article rather than being crowded in here.