I am a case worker for people with disabilities, including several people on my caseload with autism. It does depend where you are and how autism presents, even within my small caseload there is a lot of variation. Even people with “heavy degree” autism can have lot of variation. In general, my agency and jurisdiction take the person centered planning and self determination pathways; the person if able, communicates to us what they do and not want in their plan, who they do and not want caring for them, and what they want us to request (funding from Medicaid. US). In many cases, they are in command of themselves enough to have that much control.

I suppose the question you ask is; what happens to the people who are non verbal, perhaps even combative. The answer is a little sadder. The agency I work for specializes in IDD but also community mental health. Our system famously dismantled the asylums in the 90s under Reagan austerity (good) but I do think there is an infinitesimally small group of people for whom community mental health is not meeting their significant needs.

In many cases, like you said family has high needs people in their homes, until parents die. Many times, siblings become their primary guardians. Many of them are living their own lives. There exist group homes, with round the clock care. Some of them will apply for co - guardianship (that is to say, the agency running the place). It is this threshold that my agency will hand them off to another more specialized one. Medicaid will pay for staffing but not rent, so this is what occurs for families without much money.

If there is enough money, then the family might get the person their own space, so they can remain independent. In that case, my agency can petition funding for as much staffing as they need (although 24 hour staffing is pretty close to institutionalizing someone. There exist a lot of institutional barriers to funding 24 hour staffing.) Either of these combine with rights restrictions, which I have thankfully never had to institute. These are special provisions in their plans, which are reviewed by a jurisdictional authority board. Example, a lock on the knife drawer, or some kind of safety mechanism to prevent the stove from being turned on.

If they are lucky, their family has some money, they can set them up with a place. Their siblings take over as guardian and they can continue with the same support apparatus that their parents set up, with the staff handling the day to day. If they do not come from some means, then they will likely end up in a group home. They might also end up in a group home if they are too combative to live on their own.

It depends on the country.

Where I live, the heavily autistic people or other mental disorders usually move out of their parents home long before the parents die.

They get social security payments and pay rent. If they need assistance, they’ll usually choose to live in condos along with other handicapped people, where services is hired as part of the rent. This can be structured in many ways. For instance as an institution owned by the municipality, as a privately owned institution, or as a self-owning institution which is becoming more popular. Regardless of the ownership, the institution or community gets paid by the government and is subject to the same laws.

Anyway, nevermind the details, the government pays for everything in the end. The point is that handicapped people are not legally incapacitated. They have the same rights as everyone else.

In severe cases, they can be financially incapacitated. Autistic people usually don’t need to, but if so they will then be appointed a legal financial guardian. From experience, I can only stress that it is most beneficial to have a legal guardian that is NOT family. The reason is that handicapped people get a modest social security cheque to cover their expenses. However being disabled and all, they don’t really spend a lot of money, so unfortunately family members acting as guardians find opportunities to start leeching. Disgusting, I know, but it’s unfortunately what happens when they get pursuaded to let a distant cousin to run their finances…

In case you were asking about minors losing their parents, they’ll be placed in foster care. Autistic or not.

Become a ward of the state or another family member.

That will vary by country. In Germany, people can get a “Pflegegrad”, which basically defines how much assistance someone needs. Depending on that, they may be assigned a caregiver, paid for by the state. If other relatives decide to take them in, they’ll also get some money and assistance for that.