The mushroom sandwich is a moisture problem before it is anything else, and how it is solved is the whole sandwich. A mushroom is mostly water, and heat drives that water out, so a pile of cooked mushrooms laid straight onto bread will flood the crumb and collapse the sandwich within minutes. The build is therefore organised around containing and concealing that liquid: mushrooms cooked hard enough to drive off and reduce their own water, a fat that carries their flavour without adding more wet, and bread chosen and treated to resist what little moisture remains. Get the water wrong and there is no sandwich, only a soaked slice and a wet plate.
The craft is the cook and the barrier. The mushrooms are fried hot and not crowded, because a crowded pan steams rather than sears and the mushrooms stew in their own liquid instead of browning and tightening; cooked properly they shrink, darken, and concentrate, the water gone and the savour intensified. Garlic butter is the usual carrier and it does real structural work, a fat that coats the mushrooms and is absorbed by the bread as a deliberate, controlled lubrication rather than the uncontrolled bleed of raw mushroom liquid. The bread is firm enough to hold a soft, heavy filling, and a butter layer to the edges waterproofs the crumb against the last of the moisture. A raw mushroom version exists but reads thin and slips, which is why the cooked, reduced build is the one that holds.
The variations push the same idea in different directions. A garlic mushroom build leans hard on the butter and the allium; mushrooms with melted cheese use the cheese as a second seal as well as a flavour; mushrooms on toast give up the top slice so a single firm base can carry the wet pile openly. Each tips the sandwich toward a named build with its own logic, and those deserve their own articles rather than being crowded in here.