Well, I'm a little confused by your question. Do you mean what happens when you apply for Disability? (That's a FEDERAL, not state, benefit.) If you DO mean Disability (SSDI), you have to fill-out a really long application----like, 10 pages. I don't think I had to get any doctor's papers, or anything----THEY do that, going-by the information you provide on your app. It takes about 3 years----I had an uncle with Multiple Sclerosis, and it even took HIM over 2 years. Then, what the OP said is true, regarding if you want to also work, say, part-time, then they limit your income----I think it's like 1,000 dollars a month. Also, the amount of Disability you get depends on how much you've worked during your lifetime, and contributed to Social Security.
If you're talking about some state agency giving you benefits for a "developmental disability", then I don't know what, in specific, you're asking about. I know, from experience, about a very few things----like foodstamps, job search help, etc., but that's all.