How do I know exactly what does a pediatrician do for a newborn broken arm?

Refer to orthopedist. The pediatrician cannot do anything other than confirm the diagnosis (xray) and determine an etiology. Usually the newborn is referred to an orthopedist and a soft cast is placed and reassessed and readjusted frequently. Immobilizing the fractured arm should suffice to prevent the pain but the pediatrician can also prescribe tylenol (acetaminophen) for that.