Very short, commercial banks loan money from national banks. That creates money. It has to be paid with interested. Where does the money for that interest comes from? From other loans. So the whole thing becomes an ever increasing pile of loans.

And government loans from national banks to spend money, so creating and distributing money. If government pays back the loans, money disappears.

And because the whole thing is loans on loans, if population would decrease (what is starting to happen) and growth stops, loans decrease and the whole thing starts to wobble.