I don't know anything particular about the Dutch market, but some rough estimates:

Current 3m euribor is -0.4% so 1.6% interest rate means 2% margin. Assuming that your initial costs of selling/assessing/processing the mortgage are covered with fees, something like 0.5% would be enough to cover the main (non-systemic) risks and expenses for decent quality mortgages, so you get 1.5% of pure profit; for every billion in your portfolio you'd get 15 million profit each year; Dutch banks have 500B of housing loans so that's a capacity of something like 7-8 billion of profit per year from housing loans alone.

And various short-term loans have lower volumes, but much higher interest rates and room for profitability.

Looking at the stats in https://www.statista.com/topics/3442/the-banking-sector-in-t... it's clear that the net interest income dominates, it's something like 5x the fee income.

Banks usually add a flat percentage fee on top of the current interest rate. Lower interest rates mean people are willing to borrow more so low interest rates are beneficial to banks.