My quest started from the well-know but erroneous assumption that the core function of “a/an” is to determine a following noun as indefinite, as the name “indefinite article” suggests.

‘The core function of a/an’ is not to determine the following noun as indefinite but to determine the following noun as an individual entity. All of these are done in this way, and not in the order of a/an. Only after “a/an” determines the following noun as an individual entity can the “indefinite” article determine the following noun as indefinite.

Why is an indefinite article automatically precluded from preceding any plural noun and which is logical, because they are not considered separate entities?