Learn Italian: Questo non è un ristorante qualsiasi; la qualità è eccezionale. - This is not just any restaurant; the quality is exceptional.

Placing qualsiasi after the noun adds a sense of emphatic indifference or mediocrity, translating perfectly to "just any" or "an ordinary." Un ristorante qualsiasi means "just any restaurant." If we placed it before the noun (qualsiasi ristorante), it would neutrally mean "any restaurant (whichever one)."

Could I use qualunque instead of qualsiasi here?

Yes, absolutely. Qualsiasi and qualunque are completely interchangeable in this context. Saying un ristorante qualunque carries the exact same meaning of "just any restaurant."

Why do we use the indefinite article un here?

When qualsiasi or qualunque follows the noun to mean "an ordinary" or "just any," the noun requires an indefinite article (un, uno, una, etc.). If you place the adjective before the noun, the article disappears: you would just say qualsiasi ristorante.

How would this sentence change if we were talking about multiple restaurants?

It would become: Questi non sono ristoranti qualsiasi. The word qualsiasi is invariable, meaning its ending never changes, even for plural nouns. The article un simply drops away in the plural.