There are lots of ways to structure a firm. Some of them involve the sort of partnership that would effectively make the debts of the firm into shared debts of the partners. But others the debts of the firm are just that, and the only thing a debt of the firm can do to a shareholder is make the shareholder's share worthless. In the first case the partner might have to sell their Berkshire Hathaway shares to help the firm pay a debt. In the second case the shareholder doesn't have to sell off the BRK-B, or their house.