Question concerning Interest system in banking.?

These days i’m growing increasingly interested in how the Interest and loan system work at banks. Let’s say I have 1000$ i want to put in a bank, and it brings %40 monthly interest right? At the end of a month i will have 1400$ right? Ok, so here’s my question:

Where does that 400 come from? It’s created out of thin air? Printing money is illegal for citizens but not for banks? How does this work? Enlighten me please.

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