What is the plural (al-jam')?

Arabic has three numbers — singular, dual, and plural (three or more) — and two completely different ways of building the plural. The sound plural bolts an ending onto an intact singular: مُسْلِمٌ → مُسْلِمُونَ (masculine) or مُسْلِمَاتٌ (feminine). The broken plural reaches inside and rearranges the word: قَلَمٌ → أَقْلَامٌ, كِتَابٌ → كُتُبٌ. And one rule governs how plurals behave: broken plurals of non-rational things take feminine SINGULAR agreement — تِلْكَ آيَاتُ اللهِ, with the feminine singular demonstrative.

جَمْع (jam') is the plural as a category — three or more. Arabic reaches it two ways, and they behave completely differently when it comes to endings.

The sound plural keeps the singular intact and adds a suffix, so its ending is a letter: مُسْلِمُونَ / مُسْلِمِينَ. The broken plural recasts the word from the inside, so its ending is an ordinary vowel: رِجَالٌ / رِجَالًا / رِجَالٍ.

So the question is never just “is it plural?” but “which kind of plural?” — the answer decides the ending.

Why does the Quran use feminine verbs with plural subjects like “the nations”?

Because a broken plural of non-rational nouns is treated grammatically as singular feminine — its verbs, adjectives, demonstratives and pronouns all take the feminine singular form. It is not an exception; it is the system.