I'd start with making them understand that independence is good for everyone involved. Then small steps towards independence, starting with a simple part-time job. The important part here is that they see more people than just their direct family, to show them the different ways people can function in society.

If this works out, help them identify some personal goals. The hard part of being a shut-in is that you have no agency over your life, so you give up on all your goals. Once you have a little agency, you can start finding goals again. Simple ones at first: saving up some emergency funds, getting a driver's license, making a weekend trip to get out of the house for a couple nights.

Also assuming they are classic shut-ins and not caretaker-type shut-ins: learning how to take care of the household, cooking, developing skills around the house. It's low barrier to entry, they are already in the house after all.

Source: was a NEET for 5 years

Depends on how you define it. I knew a couple of guys who just grew up and did a boot camp and got programmer jobs, or took a job with their parent or friend of parents company. Also a few women, and one guy, who just married a wealthy spouse so they could continue to be moochers forever. One or two had their parents move out and give them the house while they did low-wage work.

I have never known anyone of the 'basement dweller' variety though, these are all wealthy folks from wealthy families who had tons of social connections. I met them in college/graduate school.

Also knew a couple of people who just never had a job and lived on poverty-income but from their inherited stock returns. They were into speed running games..