When you deposit $US with a US bank, in a savings account or CD, it can loan out 100% of your deposit. If banks offered BTC-denominated savings accounts, they'd work the same way. If you're thinking "but wait, that means there wouldn't be enough bitcoins in existence to allow everyone to withdraw their deposits", then congratulations, you understand how banking works.
The difference of course is with fiat currency the government/central bank can "print"* money on demand and lend it to the commercial banks so they can cover their customers withdrawals.
With BTC denominated accounts if everyone tries to withdraw their BTC at once the bank has a big problem. They can try to buy BTC to cover the withdrawals but there is no guarantee they will actually be able to.
* It starts out as entries in the central banks database but if the customers are demanding their money in cash then that cash will have to be physically printed and sent to the banks.