The Thanksgiving sandwich is the only American sandwich whose ingredient list is not chosen but inherited, and that constraint is the whole genre. It is built from the previous day's leftovers, turkey, stuffing, cranberry sauce, gravy, sometimes mashed potato, on sliced bread, which means the cook is not composing a sandwich so much as solving the problem of a refrigerator full of cold, mismatched textures. Every decision in the build is a response to components that were designed for a plate and a fork, not for being held in the hand. That is what makes it a distinct sandwich rather than just cold turkey on bread.
The craft is in managing moisture and contrast across parts that do not naturally cohere. Cold sliced turkey is the dry, neutral base and carries little on its own, so it leans on the other elements to do the work: stuffing brings a dense, savory, already-seasoned mass; cranberry sauce supplies the sharp acid and sweetness that cuts an otherwise heavy stack; gravy is the binder that turns separate cold layers into one. The structural risk is the gravy and the cranberry soaking the bread before the sandwich can be eaten, which is why the build often toasts the bread or seals it with a mayonnaise layer, and why the wet elements are usually placed toward the center rather than against the crumb. The bread is typically a sturdy white or a soft roll, sized so the leftover pile, which has no internal structure, has something firm to be braced against.
The variations are a question of which leftovers are on hand and how the wetness is handled. A dry build skips the gravy and runs cranberry and stuffing for a sandwich that holds cleanly; a hot build warms the turkey and gravy into something closer to an open-face plate; a layer of mashed potato turns it denser still. The leftover-stack logic also produces the named build that runs a separate gravy-soaked slice through the middle, which is its own distinct sandwich. Each of those deserves its own article rather than being crowded in here.