To get more flavor out of cumin, you can use whole seeds, and toast them briefly in a pan before grinding. If you don't want to put forth that extra effort, you'll just need to add more cumin. If it's not salty enough, the best solution is to just add more salt (sorry). Salt will enhance the other flavors as well. A bit of cornstarch would help make the spices stick, and dry out the mixture. I'm not sure how much water and vinegar you're adding, but you may just be watering down your beef too much. I usually add just enough to keep the spices from sticking to the pan, but not so much that there is a lot of extra liquid that needs to be reduced.
Your spice mixtures may also contain some form of MSG. You can get a similar umami "punch" by adding a dash of Worcestershire sauce.
Edit: I don't know how I missed this, but the other answers point out the lack of garlic and onion. Your store-bought spice mixtures definitely contain plenty of those in powdered form, which you can use, but fresh is tasty too. Don't add fresh garlic too early, as it can easily burn and turn bitter.
Servings/serving sizes are simply an amount that is "customarily consumed". There is no implication about what you should consume, about how much to eat per day, how often, or what part of the meal. It's just about what people tend to eat.
The FDA has put a fair amount of effort into coming up with some sort of guidelines for all this. If you care to see details, you can read this part of the Code of Federal Regulations (Title 21, Part 101, Subpart A, Section 101.12). Essentially, they have amounts for categories of foods, so that ideally if anyone makes a new product, there'll already be an applicable entry in the tables.
But obviously generalizations only work so well. There are foods in any categories where those serving sizes will be unrealistic. There's personal variation. There's variation in how large a part of a snack or meal a given food provides. There are manufacturers trying to make high-fat high-sugar high-calorie food look less bad, and picking serving sizes as low as possible within the guidelines. So you'll frequently find that they're wildly off from the amount that you'd eat for a single snack or single meal, and they'll more likely be too small than too large.
A better way of looking at serving sizes is that they're intended to be of the right order of magnitude, so that you can reasonably easily estimate the nutritional content of the amount you actually eat. For example, if your cereal is in the category with a 30g serving size, and for your particular cereal that's one cup, then whether you eat half a cup or two cups of it, you'll be able to do the math.
So for that purpose, what matters is the rough amount that people tend to eat, hence the notion of "customarily consumed" amounts. That does mean that as you say, "staples" which people tend to eat as a large part of a meal will tend to have larger serving sizes, while things typically eaten as snacks will have smaller serving sizes. But that's not a judgment about how you should eat them, just how people typically do eat them.
The parts of nutrition labels that are intended to provide some guidance about how much they think you should eat are the % daily value column, for the individual nutrients, not the serving sizes. So to see whether your mixed nuts or refried beans are an "appropriate" amount to eat, you have to look more carefully, and possibly adjust for differences between your dietary needs and the generalizations made by the FDA to pick the daily value numbers.
It's not too hard to find things where one serving provides more than the recommended daily intake of some particular nutrient, and it's also not hard to find things where ten servings would be well below the daily value for everything. So the serving size itself definitely doesn't indicate anything about number of servings to eat; it's just a sort of arbitrary but hopefully convenient unit.
That covers most of your numbered questions. As for "what is a cup"... it's a standard measurement. In US stores you should be able to find 1-cup (or fractions/multiples thereof) measuring cups. It's equivalent to 236.6 mL. It's not just the contents of any arbitrary cup, so the measuring cup that comes with your rice cooker is probably not the same - likely 180 mL.
Best Answer
It is mostly a fancy way of saying that they are combining flavors. There are no solid, physical layers involved anywhere.
Still, there is a reason why the "layering" metaphor is more apt than simply saying "combining". Flavor is mostly about aroma, which leads to two aspects of "layering".
First, aroma is not perceived all at once when you bite off. You first notice the most volatile smell notes when the food enters your mouth, go through a kind of "middle" and only at the end, while swallowing, you notice the "heavier" flavor. So, when seasoning, you can work with food such that you don't mix up too many flavors in one of the three aroma "layers", but also to make sure that there is something noticeable in each of the three.
Second, people are accustomed to some flavors being present as a metaphorical "background" taste. It can happen that, if the expected aroma is absent, you can add all the spices you want, and the eater will still experience it as underseasoned. This is what is happening in the example you cited: the "base layer" are the onion and garlic, which are probably the standard for the dish, and then you can take that dish in different directions by your choice of additional herbs and spices. In that case, you can see the metaphor as akin to clothing - once you have a basic shirt on, you can always make the outfit nicer by layering a scarf, jacket or jewelry on top of it.
All in all, it is not "all about layering flavors". It is just an expression which helps some cooks go about creating harmonious flavor combinations. If it works for you, use it. If not, learn to think about your seasoning in other terms.