MMT posits that gov’t spending can be paid for with printed money.

What is backing the printed money? It is so amazing the way a lot of countries print money just to increase the circulation of money around. Printing money continuously have done no good but cause hyperinflation which leads to a devalued currency. I heard the chinese central bank is willing to print out yen worth $100 billion to fight coronavirus, if this money isn't backed by anything, then the country should borrow from another country rather than printing excessively. It will only kill the economy in the future.