I get all of this at some level, but the piece I don’t understand is - where does the Treasury bill come from? Does the bank use its customers’ cash to purchase

I get all of this at some level, but the piece I don’t understand is - where does the Treasury bill come from? Does the bank use its customers’ cash to purchase government securities, and then the Fed buys the government securities from the bank? It doesn’t seem like that step would be necessary (why can’t the bank just create money based on deposited assets and applicable reserve requirements?), so I assume I’m not thinking about this the right way.