They must only borrow the money (or get it from somebody else who got it from somebody who borrowed it from the central bank) that they need to pay you out in paper or digital cash and the money they have to keep in reserve. They do not have to borrow the whole credit value. The credit itself is created out of thin air but with some strings attached, so banks can't just create infinite amounts of credit and they cannot just pay out arbitrary sums of created credit. (Source: Some books I read a long time ago.)