Where does the money comes from and how it works regarding ALFs and/or Memory Care living?

My mother has an annual income from SS of about $15,000. She also has money in the bank, a paid for home, and stock checks she gets dividends on, so while she is not "rich" she is comfortable financially.
So, if I find her a nice ALF she can live with (no pun), how does she pay? Will the money come out of her SS first, and then whatever is left due to the ALF comes out of her "own" money that is already in her accounts or her property?
Just wondering how the financial end of paying for mom to live in an ALF would work. Do they take money up front, like a year's worth? I wouldn't imagine they could do that with an elderly person in case they pass away, you know?
Just very confused... I mean I struggle to balance my own checkbook!