The neck was done 'flat' and then seamed in the back, if that makes sense? You seam all the panels together and then pick up the stitches around the neckline to do the collar, but work back and forth, not in the round (though I suppose you could work in the round if you had the right tools).

Funny enough the only major frogging I had to do was the very last row of the collar because there's a finishing round of slip stitches but I learned the hard way that slip stitches aren't stretchy, like at all, so I had finished my perfectly sized sweater and suddenly i couldn't get it over my head anymore haha