What is the reason for the nipple being inside?

Patient's question:

Single, no sexual experience, nipples are inside, and they feel very soft, will it affect fertility, what are the downsides

Doctor's answer:

Medical Analysis: Hello, your condition is likely to be due to nipple inversion.
Guidance: In teenage girls, nipple inversion is often caused by developmental obstruction. Generally, self-massage can be very effective.
Use your left hand to support your right breast. With your right thumb, index finger, and middle finger together, gently rotate and massage from the base of the breast towards the nipple. First, move from the breast base near the sternum outward, then massage the left breast.
Form a claw shape with your five fingers, cup the area below the nipple, and gently rotate and massage towards the breast base. Do this 10 times on each breast.
Pull the nipple. With your thumb, index finger, and middle finger, gently pinch and pull the nipple outward. Do this 20 times on each side.
Note: Apply even pressure and avoid excessive force when pinching the nipple.