Good questions!
I think the simplest description of a supplement is an 'addition of nutrients'. For example, a vitamin C supplement generally has much more vitamin C than an orange. But I would never
replace a food with a supplement because you still have to eat food! Supplements are just a good way of adding nutrients to the body, particularly if you're eating nutrient poor foods.
Protein is another fuel in the body. It helps us build strong and lean muscle mass among a myriad of other tasks. Adding protein to things helps boost the body's nutrient resources.
Soy vs. Whey proteins: Google that. It seems like there is so much information out there that it comes down to opinion. I personally use a blend of soy, pea, and rice protein.
As for replacing meats with soy, be careful. Unless there is any pressing need (i.e. you're a vegetarian or you simply don't like meat), I wouldn't do too much replacing with tofu unless you really love the stuff. A hamburger that you prepare without cheese, for example, can have a lower caloric impact and have more nutrients than a soy-based burger. Many of the soy burgers on the market have additives of cheese and other "fattening" substances which can be just as "bad" for you as eating a regular burger. On the flip side, per oz. I believe tofu tends to have more protein. Just read the labels carefully if you buy pre-made soy based foods.
Also, I think in many cases, it comes down to the method in which you prepare the food rather than which is better than the other.
Personally, I eat both tofu and traditional meats. I find that grilling tofu that I've marinated is just as tasty as any burger (and I do love burgers!) Mmmm... grilled tofu and a portabella mushroom.... delicious