How to distinguish mental illness and neurodevelopmental disorders? It looks like mental illness and neurodevelopmental disorders may look absolutely same if we superficially  observe the behaviour.
Example:

Anger, destructive behaviour can be either due to a mental illness (anxiety, panic disorder, mood disorders, etc) and also can be a meltdown that is seen in neurodevelopmental conditions (Autism, Aspergers, ADHD, SPD etc).

Stereotypy and stimming behaviour can occur due to either mental illness like OCD and severe anxiety or it may have a neurodevelopmental origin.

Anxiety itself, can be either of a psychological origin (such as either fear of something irrational, or just a state of fear), or it may have a neurodevelopmental origin such as the patient of ADHD or Aspergers or SPD can not handle a overstimulated state and may feel anxious if they have to wait for uncertain period.


There are a lot of other similarities between neurodevelopmental and psychological conditions.
Now suppose the scene a patient comes in the chamber of a General Physician/ Psychiatrist with complaints of restlessness, anxiety, depression, psychosomatic illness (nausea, headache, numbness), forgetfulness, lack of concentration etc. The general symptoms signal either a primarily psychological condition, or may have a neurodevelopmental origin. How the General Physician / psychiatrist should distinguish it?
As @BryanKrause said; mental illness and neurodevelopmental conditions are not distinctively separable.
Example: Neurodevelopmental hypothesis of schizophrenia,
Obsessions and compulsions in Asperger syndrome and high-functioning autism
Etc.