Earlier quoted context omitted.
The bank does hold a fraction of your deposits, as our banking in the US is fractional reserve banking. This has been the case for quite some time, and isn't really the same thing as digital currency, even if we transact digitally.
From my understanding, they hold most of that in reserve accounts with the Federal Reserve. But yeah, someone at some point holds some cash. But of the total US money supply (in 2018), physical currency makes up about 11% of the total value[0]. [0] https://www.businessinsider.com/heres-how-much-us-currency-t...
edit: fraction reserve _requirements_ are set around 10% or so.