«Чьи э́ти я́блоки?» is a question with an answer like «Э́ти я́блоки <мои́>» (or «Э́ти я́блоки — мои́ я́блоки»). «Чьи э́то я́блоки?» is a question with an answer like «Э́то <мои́> я́блоки».
Both mean pretty the same thing, but are different grammatically: «Э́ти» modifies the noun, while «э́то» is a demonstrative pronoun.
It took me a while thinking about this to understand the construction but I think I get it. If you were to have яблоки by itself, it'd be just apples, or the apples. So if you have the sentence чьи это яблоки then I take it to be like "whose are they, the apples?". So wouldn't "whose are the apples?" be a better translation ? If I say whose are the apples, I'd be talking about some apples sat in front of me, so these apples in particular, but the "these" is implied. Does that make sense ? Whose are these apples therefore doesn't make sense as a translation.