The shoulder piece is set onto the outer side of the front (seen from outside) and attached with whip stitching. The pic shows the front part to the left, the shoulder piece right. The front edge is at the top. Seen from this side, the shoulder appears under the front.

On the the side seam edge of the front part, the allowance is folded under and sewon onto the back with whip stitches.

The lower edge of the pic shows the armscye edge, the side seam runs diagonally towards the upper right. As the seam doesn't go all the way to the armscye, I could lift the edge of the front with forceps to show how the allowance of the front is folded under while the allowance of the back lies smoothly.

You would expect the side seam to be a backstitch seam ironed flat, but no such luck.

As in front, the back end of the shoulder piece is set onto the outside of the back lining. Seen from inside, the allowance of the back is folded under and whip-stitched to the shoulder piece.

In this pic the shoulder piece is above, the sleeve seam to the left. This also shows how wide the allowance is at the sleeve seam. See the seam starting at about the centre of the upper edge of the pic, runing downwards, then curving sharply away towards the left, leaving the pic at the lower left corner? That's the sleeve seam. Every bit of lining you see to the left of that seam is what I call, for want of a better word, allowance.

There must e a reason why the lining is cut so much wider than apparently necessary. I think it has something to do with lending support to the top fabric.