What was the term? Was it a floating rate after a lock-in period?
The thing that makes 2.5% so incredible in the US, is that it's the rate for a 30 year fixed mortgage. Get one of those loan today and you'd still be paying 2.5% interest in 2049.
I'm currently looking at various mortgage options in Belgium, between 1 and 2% for 25 year term. But what makes it even better is that, by law, (variable) mortgage rates can never more than double (in Belgium). Yearly variable mortgage rates can be had for <1%. So that means that even if it doubles in, say, 3 years, it's never more than 1.8/2%. But if interest rates remain low for years to come, I'd still get around 1% in those years (if they don't go any lower than they already are...).
I got another mortgage in the deep of the aftermath of the 2008 crisis (2010/11 or so) and I had the same situation; rates were a bit higher then. I modeled various scenarios and basically a long fixed term mortgage would only be better if rates would double after, IIRC, 3 or 4 years after I got it. Despite my instinct that screamed 'NO' at variable rate mortgages, I got one anyway - and instead of going up, rates went down, significantly (in terms of their effect on monthly payments - I'm now paying more than 10% less per month than I did at the beginning).
Much of this is completely counterintuitive. This was the first time that cold hard reasoning and financial modeling made me direct profit, and it has done so several times since, but I also have to admit that it took me years to overcome the mental blocks and feeling of uneasiness that following through with real money on decisions that are based purely on facts. I know several people who got fixed terms mortgages around the time I got my first one, and they're still happy about it, they prefer the stability - even though they rationally know there is literally no downside to the variable rate one.
All that said, the Euribor-based mortgages mentioned elsewhere where you actually get paid for having a mortgage are no longer available :)
How do negative mortgage rates even work? My brain doesn’t understand that. Does the bank make money on fees or something? -1% mortgage rate and 3% in processing fees?
Not at all. The bank is just an intermediary between the central bank and you. There are people in NL/BE/DK who actually get/got money on their mortage, monthly, just like a mortage payment but in reverse.
edit: for example, my mortage was the monthly Euribor-rate + 0.9% margin. Euribor-rate = -0.5, I pay 0.4%. But there are people who managed to get just a 0.5% margin. They pay nothing or get money back.