Nerves are naughty: asthma, dermatitis, ulcerative colitis and other psychosomatic diseases

Unfortunately, we have almost no practice when a specialized doctor (for example, a gastroenterologist, endocrinologist, dermatologist) sends a patient to see a psychotherapist, revealing a psychosomatic diagnosis. You’ll have to do it yourself.
Where does it come from?
Psychosomatic disorders are symptoms or diseases that develop as the body’s response to stress. What could it be?