A tummy tuck helps 3 problem areas of the abdomen. First is removal and tightening of the excess skin. Usually all the skin from just above the belly button down to the pubic bone is cut away. Second is the removal and/or thinning of the fatty layer. All the fatty tissue beneath the removed skin mentioned above is also cut away; then the remaining tissue, including fat in the upper abdomen is stretched downward, ultimately making the fatty layer thinner. Finally, the paired rectus muscles which extend the length from the bottom of the breast bone to the top of the pubic bone is tightened. There are various techniques, but usually an "imbrication" stitch is placed in the "fascia", or strong, thin covering of the muscle, such that when the ends of the stitch are tied, the edges of the separated muscles are brought closer together. No cutting of the muscles is performed. Personally, I place a "figure of 8" stitch made of a strong synthetic non absorbable suture that feels similar to a heavy duty cotton or polyester thread you use for sewing. It comes in different sizes, and a size that is strong enough to hold, but not so big to cause a foreign body tissue reaction is generally selected. Every surgeon has their own particular preference. Typically, those stitches are never felt below the skin. And eventually, scar tissue bonds the "imbricated" tissue, so that even if a stitch would break, the scar tissue holds the fascia/muscles together.
Some women have very little "diastasis" (separation) of their rectus muscles, and don't need significant muscle tightening. But anyone that has had at least one pregnancy will have some separation of the muscles and the only thing that brings them closer together is surgery....all the abdominal exercises in the world will NOT correct diastasis. I hope this has more than answered your question!