I'm not expert at steeking, I've only done it a couple of times. It
looks like to me he used steeks for the arm holes and the neck opening.
Basically knit a tube putting in the shaping where it needed to be
and then cut to open up the arms and neck then went back and
picked up stitches around these openings and knit the ribbing. For
an exact how-to I better let a more experienced steeker answer!