Reddit mentions of Dopamine Handbook

You are asking about a very broad topic. Books are written about the role of dopamine in the brain.

For the specific case of ADHD, methylphenidate (Ritalin) acts to increase dopamine and norepinephrine by blocking reuptake - meaning it does not work by binding directly to dopamine receptors. Because of NE, its actions are predominantly as a sympathetic stimulant rather than a dopaminergic agonist.

For schizophrenia, you can see a reduction in positive symptoms (delusions, hallucinations, loose associations) with typical antipsychotics that are thought to act as antagonists at the D2 receptor. We don't have any drugs that significantly improve negative symptoms (dissociation, flat affect, etc). This receptor is also though to mediate the extrapyramidal side effects - including disorders of movement. If you've ever seen a patient on these drugs, you'd know what I mean about sedating - sometimes they can barely keep their eyes open.

The newer atypical antipsychotics seem to work more through serotonergic (5-HT2) receptors, with a great example being clozapine, with almost no activity at the D2 receptor, but strong activity at 5-HT2 and mild affinity for cholinergic receptors. Consequently, clozapine has minimal extrapyramidal symptoms and may even improve tardive diskinesia. Of course, with these drugs, you have significant weight gain as a side effect.

So, suffice it to say, the role of dopamine in normal functioning, disease, and treatment is complicated and can't be easily summed up in a reddit post - and certainly not as in the top post above.