Other countries and banks use their money too. If a bank suddenly has its hands of a billion Imaginationland dollars it might exchange those dollars in to another currency it wants more by trading with a corporation that intends to use those Imaginationland dollars to make payments on a contract they have with a company in Imaginationland that needs to be paid out in Imaginationland dollars.

Or the bank might use those dollars to buy shares in Imaginationland corporations listed on the Imaginationland Stock Exchange, which would result in those billion dollars going in to the accounts of the Imaginationland corporation that issues those stocks for sale and then they'll use it to pay salaries, pay out vendors, etc

It'll always find its way in to circulation somewhere.