The fronts are sewn to the back, and the button placket band was worked separately and sewn on. I think I will rewrite the pattern so that the button placket is included during the knitting of the fronts, because I find a separate button placket hard to wrangle, and then there's the matching up of the button markers and buttonholes, and making the seam come together at the back of the neck... just, ugh. No thanks.
The first picture is the first sleeve in progress. It also features that pretty cable pattern.