I'm a care worker. I don't do live-in care, but I know several people who do.
There are umpteen different permutations to live-in care, there really isn't a 'norm'.
Quite frequently, I cover the live-in carers' break times - which is usually 2hrs a day.
So, there will be one carer who resides with the client permanently - but, for two hours a day, she can go to her room, or go out for a walk or whatever - and I just hold the fort while she's not there.
Anybody employing a live-in carer should be able to offer exclusive use of a bedroom with some basic storage, like a chest of drawers for eg.
They should also have access to a bathroom, although not necessarily exclusively.
Live-in carers choose this way of working for a number of reasons - generally speaking, they have no immediate family responsibilities (like children), and they can earn much more than they could in a different setting such as domiciliary care.
As with anything in life, the standard of carer can be wildly variable.
Often, care agencies can source you a live-in carer, carry out all the necessary checks, and deal with all the administration.
They often have a database of live-in carers, so can find you someone at short notice. They can also deal with any issues, so that you don't have to.