Yes its like when the customer/depositor gives the bank 10K they can now lend out 100 grand
This is my understanding. Some of you are assuming that the bank must have at one point had 100% of the 90% they lend out.
I'm not sure this is the case. My understanding is that they are only required to take in 10% of what they lend out, not that they had to have had the 90% they lend out in the first place.
This means, as Smaulgld says above, that the bank can make 100 grand worth of loans just from a 10,000 dollar deposit. They need not have ever possessed that 100,000 that yes, they can just create out of thin air, and then make interest on, without ever having that 100,000 in the first place. It is as simple as clicking the button to put funds in someone's account that never previously existed (well, 10% did, but not that 10x that they lend out).
What enables them to so? Well, having a federal reserve with the power to print money out of nowhere certainly helps, and this same federal reserve is there should this Ponzi scheme fall apart via a "run". If it were actual gold* they were loaning out (or whatever tangible backed currency) and/or they had to have these deposits in the first place, then this would not be so possible, nor unacceptable if they were taking on risk on actual money that they'd taken in, rather than having the Fed to bail them out with more fake money.
(*Note that even when dollars were backed by gold, these "promisary notes" were still not gold they were giving out)
This is certainly even easier now that all banking is largely electronic, and very little done in cash, and virtually none in gold (thus meaning that even a gold standard would not be immune from this practice). When they make a loan, it doesn't have to mean anything more than a click of a button that enables their check or credit card.