"By the way, what do you do?"
Translation:Nebenbei, was machst du?
I have been running circles on the internet trying to help answer your question and unfortunately, the only thing I came up with is: for both question words and adverbs at the beginning of a sentence, the verb comes after each and is in a 'second position', even when a sentence contains both!
There isn't a subordinating clause or anything, and in fact, I am kind of confused as to why there is even a comma in this German sentence.
Sadly, I wish I could point you to a link to show you some concrete word position order formula for this, but you'll have to take my word.