It's interesting that the OP was asking about over cooking base and we ended up with a feeling that some are actually under cooking, and that there is (or could be) an adverse reaction to having extra vegetable ingredients. My feeling about the cooking is; if the cook thinks it's done, and they're happy with it, that's it. If it makes a good curry to their liking, that's what matters.
As for the science of it, I'm sure that onions generate sugars when cooked out to perfection. I can only hope to achieve this from time to time and since I'm only doing home based small batches, it may or may not happen.
Do they add spices? You bet. Will they tell you what they use? Partially.
I have a Greek friend who at 55 years old found out his father's recipe for Lamb Souvlas, and only after his mother passed away. Will he tell me the whole thing? Nope, and I even gave them some of my home made Ouzo. Are they good? You bet.