In the old days, it required actual money. Then it requires the trust of the public, plus enough reserves to  be able to react to unexpected cash movements.

Nowadays, with regulatory oversight, it requires a capital allocation (8% or less, depending on the risk).

Not that different from a car, which only really requires the leasing payments or whatever scheme the rental company is using to have access to it.

In the long run, we're all dead. John Maynard Keynes