Learn Turkish: Çorbaya avuç avuç kimyon ve tarçın koyuyor muydu? - Was he putting handfuls of cumin and cinnamon into the soup?

This is an ikileme (reduplication). The word avuç means a handful. By repeating it, the sentence emphasizes that large, repeated quantities were used, translating to "handfuls of" or "by the handful". This is a common way in Turkish to express abundance or a repeated manner.

Why does the soup take the dative case (çorbaya) instead of the locative case (çorbada "in the soup")?

In Turkish, the verb koymak (to put) implies movement towards a destination. Even though in English we say "put in the soup", Turkish focuses on the direction the spices are traveling. This requires the dative case suffix -a (meaning "into" or "towards"), combined with the buffer letter -y- because çorba ends in a vowel.

Since avuç avuç implies a plural quantity, why aren't the spices pluralized?

In Turkish, whenever you use a quantity word or expression that implies "more than one" (including reduplications like avuç avuç or numbers like iki), the noun that follows it remains strictly in the singular form.

How is the verb koyuyor muydu constructed to mean "was he putting"?