You deposit $1000. The bank does the following accounting:
accounts["thehappypm"] += 1000
accounts["reserves"] += 1000
Then I go to the bank and request a $900 loan. It knows it has $1000 lying around doing nothing useful so it just gives a fraction of it to me as a loan. The bank does the following:
accounts["reserves"] -= 900
accounts["matheusmoreira"] += 900
So now we have three accounts:
accounts["thehappypm"] == 1000
accounts["matheusmoreira"] == 900
accounts["reserves"] == 100
So the bank received a $1000 deposit, kept a $100 reserve and loaned out $900.
That $900 is completely made up money. They just credited my account with that amount out of nowhere. I'm supposed to go out there and earn money to pay it back. If I do, all is well. If I default? The bank has its books showing they owe you $1000 but they only have $100 in reserve. The illusion is shattered the second you attempt to withdraw that money.
In the real world, it's not just you and me. It's banks leveraging their massive reserves on risky loans and investments that fail, thousands of customers noticing and attempting to withdraw everything they have all at the same time, getting nothing, leading them to default on their debts and so on and so on until the government literally manufactures even more money to bail out the banks by "injecting liquidity" into them and restoring their reserves, putting an end to the bank run at the cost of even more inflation.
So I decide to spend that $900 on a computer for software development in order to work remotely. I go to the store and buy one.
accounts["matheusmoreira"] -= 900
accounts["store"] += 900
accounts["reserves"] += 900
accounts["thehappypm"] == 1000
accounts["matheusmoreira"] == 0
accounts["store"] == 900
accounts["reserves"] == 1000
The bank just received a $900 deposit, so its original reserves are restored. This means another loan is possible! This cycle repeats itself many times as money circulates until millions have been created out of thin air. Notice that the only way those accounts balance is if I pay back my $900 loan. If I lose my job before that happens? They won't balance and the bank will have to pick and choose who can withdraw and how much.
Not too long ago, Binance had restrictions on bitcoin withdrawals. Unverified accounts could only withdraw a small amount per day, verified accounts could withdraw a larger but still limited sum. Hmm...