"Are they sitting?"
1st and 2nd person singular and plural can be left out (minä, me, sinä, te), but 3rd person singular or plural cannot (in ordinary sentences anyway). The biggest reason for this is that there's a high chance of confusion as to who's doing what if the pronouns are left out.
"hän istuu" - she/he sits
"se istuu" - it sits (spoken Finnish for "he/she/it sits")
"he istuvat" - they sit
"ne istuvat" - they sit (animals etc.)
(spoken Finnish "ne istuu" - they (people or animals) sit)
The question word, in this case "istuvatko" because the verb "istuvat" has the -kO question ending, comes first.
"Missä he istuvat?" - Where are they sitting?
"Miksi he istuvat?" - Why are they sitting?
"Hekö istuvat?" - Are they the ones sitting?
"Istuvatko he?" - Are they sitting?
Let's ignore the question ending for right now and make them, "Istutte vs Istuvat". Both words are have the verb stem sit, "istuu". The first word, "istutte" has the second person plural ending so it means "You (all) are sitting" or "You (all) sit". The second word has the third person plural ending so it means "They are sitting" or "They sit". If you add the question endings, the "You (all) are sitting" changes to "Are you (all) sitting?" and the second word changes to "Are they sitting?"