Usually, the meats go on the bottom and the toppings go on the top. Same for burgers. From the bottom up, my favorite sandwich is bottom bread, turkey, salami, tomato, lettuce, and sour cream and onion chips, then the top bread. It’s always in that order.
Two slices of bread, side by side. Cheese on one slice. Lay down slice of turkey across both pieces. Cheese on the other slice of bread, on top of the turkey. Unless turkey is thick enough, add another slice and close sandwich to fold. Ope, and add lettuce, salt, pepper, and vinegarette.
Iirc there is actually a reason why the order should be as so:
Bottom bread with mayo
Top bread with mayo
You can obviously substitute stuff, like something besides Mayo. Anyway, the idea is to get the meat closer to your tongue so it's the first thing you taste after a bite, then all the other elements incorperate in. Additionally, putting lettuce and tomato under the meat would cause the bottom bread to get soggy because it has more weight from the meat being on top, you don't want it falling apart on you, so the meat goes on the bottom.
I start with Butter, Mayo or Aioli on the bread/roll, then put the meat on the bottom piece and spinach/lettuce on the top piece. The order for the middle stuff doesn't matter too much. I usually start with Tomato on the meat, add salt & pepper, then some pickles/beetroot/whatever else I have in the fridge. Whack the halves together and om nom away.