Baguette Garnie is less a sandwich than a generic French term for one. Literally "filled baguette," it is what a Parisian asks for at the counter when they want a sandwich on a baguette and the specific filling is either a question for the menu or a matter of indifference. It is the umbrella under which the Jambon-Beurre, the saucisson sandwich, the brie-and-butter, the rosbif, the saumon-beurre, and every other baguette-format French sandwich technically lives. As a menu item on its own it usually means: a half-baguette, butter, a filling of the cook's choosing on a particular day, and whatever the boulangerie has in the case.
The structural logic is the structural logic of the baguette itself. A fresh baguette gives you a crackling crust against a soft open crumb, and the sandwich exists because someone realized a long sliced loaf could carry a filling neatly along its length without compressing the contents. The Parisian baguette tradition is about ninety centimeters long, weighs roughly 250 grams, and is meant to be eaten the day it's baked. A baguette garnie made on a stale loaf is the kind of thing that gives French sandwiches a bad reputation in airports. A baguette garnie made on a baguette pulled from the oven that morning is the thing that gives the format its reputation everywhere else.
The variations of the baguette garnie are essentially the entire French sandwich repertoire, which is why this entry exists more as a category-marker than as a sandwich in itself. Each filling that has earned a name gets its own treatment: ham and butter goes to Jambon-Beurre, regional cheeses go to Baguette Fromage, cured sausages to Sandwich Saucisson, fish to Baguette Poisson, and so on. What "baguette garnie" tells you, when it appears on a menu without further qualification, is that the cook is keeping their options open. The bread is the constant. The filling is whatever fits today.