One way to generate interest is to simply park it at the Fed. Check out The Narrow Bank for the story of an upstart that wanted to make that their whole business model.
https://johnhcochrane.blogspot.com/2018/09/fed-nixes-narrow-...
https://johnhcochrane.blogspot.com/2018/09/fed-nixes-narrow-...
Yes, it is initially cash. But if the bank wants to make any money, it needs to convert that cash into some other vehicle for generating interest.