when you deposit 100$ into a bank account your balance shows up as 100$ however the bank loans a portion of the money you gave them to personB for example 80$. Now personB has 80$ in their balance and you still have $100 in your account. Now there is 180$ more in circulation, the bank created $80 US dollars out of no where.
but it gets worse, personB uses his $80 loan to pay personC. PersonC then deposits this $80 into his bank... and the cycle continues over again. And that's only one way of how USD is created.