The banana and custard sandwich is unusual among sweet sandwiches because the wet element is also the structural one. Most fillings that bring this much moisture would destroy the bread; set custard does not, because once it is properly chilled it stops behaving like a sauce and starts behaving like a soft solid. That is the whole trick of the form. Thick custard, cooled until it holds a shape, is spread on buttered bread with sliced banana and pressed, and it sets into a sliceable cushion that carries the fruit rather than soaking the crumb the way a loose custard would.
The craft is about that set and the order of construction. The custard has to be cooked thick, well past pouring consistency, and chilled fully before it goes near bread, because warm or thin custard runs straight into the crumb and the sandwich collapses before it is cut. Butter spread to the edges is the second line of defence, waterproofing the bread against whatever moisture the set custard still gives up and against the banana, which weeps as it browns. The banana is sliced rather than mashed so it sits in distinct rounds within the custard instead of thinning it, and the sandwich wants eating soon after building, because the banana's oxidation clock runs here exactly as it does in the plain version. The bread is soft and plain so it presses to the custard rather than fighting a filling that has no chew of its own.
This is a build with few codified variations, because the structural demand of set custard does not leave much room to change. A spoon of jam under the custard turns it toward a trifle on bread. Banana custard made with cold-set instant powder rather than a cooked pouring custard is the everyday shortcut and behaves much the same once chilled. Those deserve their own article rather than being crowded in here.