As I can see, those seams are hand pinked as was common in 19th century bodices, and also whipstitched on most of them.

MANY Victorian bodices were made with exposed seams. This was the long-running, contemporary dressmaking technique. Although, occasionally we will see extant bodices with a full lining attached to cover the inside seams, boning, closures, etc. But it wasn’t common yet. You can see examples and read more of both in my Linings & Underlinings post here.

Hei.

Was it just two layers used sew together as one piece of fabric?, no lining that cover-up all the seams and boning?, etc

I would be an older technique but still viable. I’ve not seen it used much at all that late in the century as sewing technology and methods of construction had changed. If you are wanting to go more historically accurate I would recommend flat fell or French seams instead.

To finish the seam allowances, the hand stitches are more like binding the edge (which includes the fashion fabric and underlining too). If the seam allowances are tacked down to the bodice on the inside, then yes, the stitches only grab the underlining fabric.