It's entirely reasonable to be confused at this point, because words change their function depending on how they are used in a sentence.
For example, you could make a good argument that "a giant collection" of marbles is not a focus, it's a quantifier: one marble, 100 marbles, a giant collection of marbles—aren't they all quantities?
Sounds completely reasonable! And it's hard to explain why it's not correct. The easiest way to understand is to try playing around with components of the noun group. Which of these sentences sounds wrong?
- A giant collection of marbles rolling down.
- A giant collection of one hundred red Sharp Shooter marbles rolling down.
- One hundred a giant collection of marbles rolling down.
The last one breaks because "a giant collection of" needs to go at the start, where it functions as a focus.
If you're ever confused about word groups, just add and subtract components and move them around, and if you are a fluent English speaker you will very quickly get a sense of what is right and wrong.