symmetry: the correspondence in size, form, and arrangement of parts on opposite sides of a plane, line, or point; regularity of form or arrangement in terms of like, reciprocal, or corresponding parts. One half of the map is a mirror image of the other half.
That's not what he was asking. So, for a symmetrical map, there isn't a way to copy one part and "paste" it on the other side of the map. So, you are going to need to try your best to make it exactly the same on the other side.
the canvas of foundry isnt actually symmetrical down the middle believe it or not, if you count the squares from the middle, the left side is half a square larger, so if you want to make a map perfectly symmetrical, then you must go from the sides in, and also not include the back areas of foundry, and only the square of foundry, and also using markers, like one box up and 3 doors wide from the 3rd floor square from the corner. its very time consuming and difficult to do proffesianally. think of the grid system, like left x amount of (insert object or set length), then up (insert object of set length,) from the both sides of foundry x amount of squares from something.