What causes the joint between clavicle and sternum (actually the upper part of sternum) to protrude? How to improve?

The sternoclavicular joint itself is more prominent, and the thin man is more prominent. Leave it alone if it doesn't hurt or itch. Totally normal.

The clavicle is S-shaped, not a straight stick. Don't worry.

There should be a dent in the upper sternum. Since you have a protrusion, it may be deformed, but there are no important nerves and blood vessels passing through the notch on the sternum, so I think you are more likely to have no symptoms. If you don't feel sick, don't worry. It's not too late to correct the symptoms. Good luck!