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Particularly: “These two terms (noun and particle) appear to be of separate derivation. The particle seems like it might be Sino-Korean from 達, same as Japanese 達(tachi, pluralizing suffix).”
Korean does not grammatically distinguish between singular and plural nouns. Thus, while 사람들 (saramdeul) means "people", 사람 (saram) can mean either "person" or "people", depending on context.
들 (deul) is rarely used with nouns denoting inanimate objects. It is more often used with nouns denoting animate objects (people and animals), but then only when it is semantically necessary to make a distinction between singular and plural, or to emphasize plurality.