I get SSI and SSDI for Autism (then I became my own payee at age 25). Why did my mom teach me you get it just for having Autism?

The answer is because your mother is ignorant.  It's just that simple.

You DO understand that Social Security Disability is awarded if the applicant can't work due to mental health issues, right?

Your brother is either a few grades "lower than" you OR you dropped out of school.  It can't be both.

SSDI has nothing to do with the ability to work.  Ask someone to explain it to you.

Social Security benefits are awarded to adults when they have a disability that prevents them from being able to work substantially and gainfully.  Social Security's definition of disability is different for childhood disability benefits through SSI, thus an entirely different standard is applied.  That's because the ability to work cannot be the standard for determining childhood disability since children are all generally incapable of working substantially and gainfully and because child labor laws prevent anyone even trying to have children work at all, much less substantially and gainfully.

Apparently, Social Security's Disability Determination Service decided based on your medical records, including possibly a consultative examination paid for by Social Security, that your disability met the legal standards for preventing you from being able to work substantially and gainfully, which isn't just getting a job but getting a job and keeping that job by being able to continually work it successfully without special help and earn at least $1,260 a month. Social Security, in their expert opinion, found that you are not capable of doing that based on your medical records. Also, apparently, Social Security did not find your brother is not capable of doing that based on his medical records.

Now, one thing to consider is the type of autism. There are three types of autism: classic autism (a.k.a., Kanner's syndrome), PDD-NOS autism, and Asperger's syndrome. You and your brother may not have the same type, and that can make a difference.

People who have a diagnosis of classic autism, which called that because that was all autism used to be until other developmental disability got included under its umbrella, and people who have PDD-NOS autism meet what Social Security calls a "listing," meaning if you have the diagnosis, you are automatically deemed disabled and unable to work. That is because people with these two types overwhelmingly don't have the ability to work substantially and gainfully, so it saves the government time and money of having to develop evidence of functional capacity for every single case when they all, or nearly all, come out the same, indicating an ability to work substantially and gainfully.

It's not the same with Asperger's syndrome autism. With Aspberger's there is a spectrum that goes from high functioning to low functioning. So with people who have Asperger's syndrome, Social Security must develop evidence of functional capacity to rate the severity, to determine where the individual falls on the spectrum from high-functioning to low-functioning as it has been found that many people with Asperger's who are higher functioning do have the capacity to work and earn substantially and gainfully.

What this means is your brother must have Asperger's and Social Security did not find evidence that it reduced his functional capacity to the point that it prevents him from being able to work substantially and gainfully, whereas you must have Kanner's syndrome, PDD-NOS, or a form of Asperger's that is lower on the spectrum than your brother's.