Banks operate on cash flow.
If the national bonds are priced at -0.6%, banks simply need to prove future cash flow, ergo payment at x year. This allows them to yield the interest on the negative-interest national bonds. Easiest way to prove future cash flow is to create debt by punching in numbers into a screen for a big-ticket secure-value asset; homes are the go-to for this.

In the end, the government prints surplus money to cover the bank's profit. What profit? If the national bond is at -0.6% and the mortgate is -0.5%, the bank makes 0.1%. The more cash flow, ergo debt, it can create, the more money the bank makes.