This is the best video I've ever seen that shows how a fine professional costume is built.
Look for the patterns pieces made of heavy craft paper with no seam allowances.
The stitching lines are drawn onto the inner most layer of the bodice, then the layers are hand basted together before assembly. There are probably 3 layers, the outer layer, a layer for padding so bones won't show through and the inner layer that has the bone tape with the bones sewn on. We used to lay the layers over a large cylinder when pinning them together so each piece would conform to the shape of the torso to prevent bubbling. Costumes like this are usually worn without a separate corset, unless the actor has to get undress on stage and it is necessary to see the period correct under garments.
When they are dressing the actress you can see the built in corset. It laces in the back, but then the lacing is covered by a back panel. And there are adjustable shoulder straps in the front under bodice that won't show when the dress is worn, but you can see them as the dress is being put on.
Also, notice the side seams of the bodice are serged and total exposed on the inside. This is to make the dress easy to alter.
See the way the pleating of the skirt is done on the dress form over all the petticoats. I used to do that when working in the professional shops. Or ... actually ... since I was an assistant draper at that time, the draper I worked for would pleat one side and then tell me to make the second side exactly the same ... which was REALLY hard some times. Those skirts have so much fabric in them. Watch how the skirt moves when the actress wears the dress at the end of the video.
OH, I loved seeing this. Theater costumes are amazing to me. The people who make them are SO skilled!!!