There's no helping verb here in Swedish, it's just the normal question word order. In all direct questions, the verb is moved to before the subject. Normal word order in a statement is Du förstår. 'You understand'. To turn it into a question, we put the verb before the subject and get Förstår du?
It's the same in questions that have a question word in them, like var 'where', vem 'who' etc.
Statement: Jag bor här 'I live here' – question: Var bor du? 'Where do you live?' – the verb goes before the subject.
Ja, det förstår jag is not a good answer to this question, but Ja det gör jag would be. I wrote about that structure in a comment here: https://www.duolingo.com/comment/5944292