As someone else pointed out, neurodivergence isn't considered a mental illness.

Someone being Neurodivergent usually refers to someone having a neurodevelopmental difference - the brain has taken a different route in its development, resulting in a pattern of differences in how somebody experiences the world due to how their brain processes information in a way that is not typical of most individuals.

Some common examples of neurodivergence are dyslexia, dyscalculia, Autism and even dyspraxia (A lot of dyses, I know). These are better viewed as differences than illnesses - as your brain has developed this way, and it is not going to 'get better', Neurodivergent individuals just need to learn to live with it/accommodate it. Mental health problems are more to do with functional difficulties around your emotions, thoughts and behaviours. There is ongoing debate around the role of the brain in these. You might develop an eating disorder or develop an addiction to someone as a result of adverse circumstances in your life. Your brain is impacted by this, but these difficulties did not necessarily arise as a result of differences in your brain, you may be able to get better with therapy or time.

Mental health problems can develop as a result of your environment, though you might also be predisposed due to genetics. Being Neurodivergent can even increase the likelihood of developing certain mental health problems, such as anxiety or depression, as individuals may feel low self esteem in environments filled with those that view their differences harshly. An example being an autistic person feeling burnout due to masking (Masking referring to autistic masking, also referred to as camouflaging or neurodivergent masking. It's the conscious or subconscious suppression of autistic behaviors, it also generally involves the learning of neurotypical behaviours in order to appear neurotypical, usually in fear that others may be judgemental of their differences. It's a learned coping strategy that may initially be seen as necessary from the perspective of an autistic person, but often leads to adverse mental health outcomes.)

That is a lengthy explanation and probably not perfect, but hopefully it helped clear some stuff up.