"There is a big selection of glasses in this store."
Translation:Il y a un grand choix de lunettes dans ce magasin.
This one is a bit peculiar because "il y a" is a bit of an oddity.
The Subject of this sentence is actually "Il". Apart from oddities like Imperatives and questions using verb inversion the Subject of a sentence pretty much always immediately precedes the verb (including its accessories like optional negation and optional object or adverbial pronouns, in this case "y").
The Object of this sentence is "un grand choix de lunettes", which is known as a "noun phrase".
The phrase breaks down into:
- A determiner: the indefinite article "un"
- A qualifier: the adjective "grand"
- The noun: "choix" (masculine singular)
- Another qualifier: the prepositional phrase "de lunettes"
The latter consists of the preposition "de" and either the noun "lunettes" (feminine plural) or the noun "verres" (masculine plural).
One noun can never agree with or modify the gender of another noun (although nouns can be combined into a new noun, eg "un sèche-linge", and there are specific rules for that).
One noun can only qualify another noun through a preposition (by, with, from, to, at etc.) and that does not require any agreement nor affect any gender.
Hopefully this breakdown helps you to understand why "lunettes" is qualifying "choix" and not vice-versa.
Note that if "lunettes" was also qualified, any adjectives would have to agree with "lunettes" and not with "choix":
"a large selection of lightweight glasses" → "un grand choix de lunettes légères"
But if it was qualified by another noun, there would be no agreement:
"a large selection of designer glasses" → "un grand choix de lunettes de marque"
"a large exclusive selection of lightweight designer glasses" → "un grand choix exclusif de lunettes de marque légères"
"a large exclusive selection of inexpensive lightweight designer sunglasses" → "un grand choix exclusif de lunettes de soleil de marque légères et peu chères"
(I expect Sitesurf will have to correct that last one!)