Usually new money is created when the central bank issues it in exchange for government bonds. It's practically always possible to do this. You could posit a theoretical situation where government bonds are literally indistinguishable from money, but that just means that government bonds are money, so the government treasury has merged into the central bank. Then the central bank has to buy something else, which means it incurs some risk. Or the government can issue more government bonds. "Spending money in the economy" is like this, assuming that the government bonds are permanently rolled over, and never bought back in full.