why is яблоки the plural for Accusative яблоко?
The rules as I've learned them so far say that, for inanimate objects, we use the Nominative Neuter Plural ending, which is -а for neuter words ending in -о. So, the plural should be яблока, not яблоки.
Of course, яблока sounds exactly like яблоко when it is pronounced.
Or "Вы едите яблоки?" said to a group of people, "Are you [all] eating apples [at the moment]?"
NB: I never translate formal "you" in any language as "you all" because Duo will invariably reject it. Duo gets to use it, to make clear that it wants the 2nd person plural of "you", but we users don't get to use it.
You mean definite "article", as in "the apples" - but with Russian, there's no definite article, so without some other context, it's impossible to say whether this sentence is talking about "apples" or "the apples" - although I suspect that there is a way of making this difference clear in Russian. So far, it seems that using some form of это would be the way, but I'm sure there are more.