Questions

It's a variation of a typical mitred-corner that shirt makers do. The more conventional way follows:

From the wrong side, open up the placket and fold up and inwards its corner by 45 degrees, so that its bottom edge meets the vertical crease of the placket. This has to be done before the typical 1/4 inch edge-stitch along the vertical length of the placket.

Press the fold then pin/baste it in place to ensure that the edge remains secured in the fold. Edge stitch along the diagonal fold. Next, conceal and secure the folded-up edge when doing the 1/4 inch edge stitching along the vertical length of the placket.

Make some samples before doing this on an actual shirt!

The photo you posted shows a slight variation. By the look of things the whole placket is edge-stitched. I leave mine 'open'. To reduce bulk, I cut away the interfacing below the 45 degree fold. I also don't sew the hem all the way into the placket. I cut away some of the 'free' allowance there, which also helps to reduce bulk. The following shots are from an Italian collar shirt, so the placket is slightly different. Same principles apply though (just ignore the bias binding and imagine instead the additional fold of a French placket.