My point might be wrong but as far as I could understand by now they usually use उसका -type pronouns (or whatever starts with उस..) to express singular reference (like "he" or "she"). And उनका (or whatever starts with उन..) is used in cases when we talk about plural reference (like "they"). Maybe someone more experienced can share their point.
Well, it literally translates to - Their apple But that's incorrect. उन is used for more than one person (plural form) उस would be the singular form. के here is plural whereas, का would be the singular form. Adding का or के here changes the no. of objects that are apples in this case. Hence, उनका सेब = There's an apple that belongs to more than one person and it would translate to Their apple. उनके सेब = There are many apples that belong to more than one person and would translate to Their apples.