I don't think banks are borrowing any money (reserves). Banks borrow reserves from other banks or the central bank when they are lending and they have not enough reserves.

In the current situation the central banks has been buying assets from the banks in order to increase the reserves available, so banks have a lot of reserves and don't need to borrow. In fact, I don't think they know what to do with so much liquidity because there are not enough demand of credit in the economy.