Learn Turkish: İşte kim var?

The word for work is iş. The suffix for "at/in/on" is usually -da or -de. However, because iş ends in a hard, voiceless consonant (ş), the d in the suffix changes to a t to make it easier to pronounce. This is called consonant assimilation. So, iş + -de becomes işte.

Why do we use var to ask "Who is at work"?

In English, you use the verb "is". In Turkish, when asking who is physically present or located somewhere, it is more natural to use var (there is / exists). So, İşte kim var? literally translates to "At work, who is there?".

I learned that işte means "here it is" or "voila". Is that the same word?

It looks exactly the same, but it is just a coincidence! The exclamation işte ("here it is!") is a fixed vocabulary word. In our sentence, işte is grammatically built from the noun iş (work) plus the locative suffix -te (at). You can always tell them apart by the context of the sentence.

Could I change the word order and say Kim işte var?