The muscles that are addressed when the plastic surgeon tightens the abdominal wall are the rectus abdominis muscles which reach from the chest to the pelvis. They are two parallel muscles that are not actually joined, but run close to each other in the normal situation.
Often, after pregnancy or weight gain followed by weight loss, these paired muscles are separated as the stretched abdominal wall does not always shrink back to its previous condition.
During TT surgery, these muscles are "plicated," that is the tissue lining the muscle are sutured back closer together. The muscle fibers of the right and left muscles are not actually touching one another. It is the lining tissue called fascia that heals together, not the actual muscle tissue.
If later abdominal surgery is done through the plication area, those tissues would be sutured back together again to restore the contour as it was before that abdominal surgery.
Thanks for the question, best wishes.