Now you made the edit, it is clearer to me what you ask. Here the answers to your two questions:
2) They don't need any agents. Protein acts as a glue when heated. Protein is almost the only food type which will actually "glue" things together - try making gluten-free pancakes with non-protein flour like almond flour and you will notice what I mean. (You can "cheat" and use hydrocolloids like xanthan which make a mass so viscous that it sticks together, but that's another topic). In other, rougher sausages, you can notice the difference between the lumps of stuff, because the lumps are bigger. Some rough sausages can also be likely to fall apart, but that is because they are not cooked, they are cured raw. Hot dog sausages are made from animal protein, animal fat, flavor agents (not too many), and fillers. The amount of meat is enough to cause the mass to curdle in a tight sausage, just like the proteins in the egg yolk are enough to cause a creme brulee to curdle in a holds-its-own-shape custard. You can cook your own sausages at home, and they will bind without any additives.
The cohesiveness of a hot dog can be achieved with a relatively low amount of meat when compared to the fat and the possible fillers (like bread). If a producer decides to use yet more fillers, he can use transglutaminase to get the tight texture even when there is less meat than normal in the sausage. I don't know whether producers actually do this or don't, I suspect that at least some do it. It is possible that in some jurisdictions, they are not allowed to.
1) As I already mentioned: the hot dog is made from meat, fat and a few fillers. It doesn't contain much seasoning. The meat used for it is low-quality meat, pure uniform protein from young animals, mostly pigs or chickens. And it is boiled, not exposed to direct high heat. Therefore, the aroma component of the flavor is almost completely missing. Other meats you eat have their own aroma, depending on the animal and developing with age (have you tried eating old goat?). They also get that lovely seared aroma from caramelization and Maillard reactions at temperatures which aren't achieved through boiling. Both are missing from a hot dog. (The second is missing from other sausages too, but good sausages such as cured salami or lukanka are made from aged beef or donkey, and they use generous additions of herbs and spices, often also smoke). You just get a faint aroma of uniform, underdeveloped protein.
The taste component of the flavor doesn't contain much either. There is salt from the seasoning, and there is some umami from the meat (but not much, this being young meat). Don't forget that while most people like umami, it is more known for enhancing the aroma of the food than for being a consciously pleasant flavor on itself the way sweetness is. Parmesan, tomatoes, meat - they are tasty because of the aroma they have, not just because of the umami per se. Pure MSG is very umami, but very disappointing if you try to eat it by itself. The fat in the sausage acts similarly as a flavor enhancer. Both the umami taste of pure protein and the fat would be great if they had something to enhance, but there isn't anything in the hot dog to be enhanced, so the flavor stays bland and uniform.
The veggie hot dog producers can use this well. Almost-pure soy protein is not that different from almost-pure chicken or pig protein where taste is concerned. The texture is different, but processing both to a fine paste with the right amount of moisture takes care of that. Neutral fat can be added for mouthfeel without changing the taste. In theory, the protein of the soy should be able to bind the sausage upon curdling. In practice, I suspect that the extraction process which purifies the soy protein from the bean-tasting plant matter curdles the protein and they use some kind of additives to "glue" the sausage afterwards. (At least the pure soy protein I have had as meat substitute was already curdled. Maybe there are other processes which don't require that).
So what is a hot dog sausage? Meat processed to the point where it is almost pure protein, fat processed to the point where it tastes neutral, salt. Some starch to improve texture. Everything cooked together in a sausage shape. Doesn't sound too appetizing, but it is cheap, filling, and the taste attack of mustard and ketchup is so strong, that any better sausage would be wasted in a hot dog sandwich.