Studded tires. This is an investment but it works. Caveat is you need space to store them on a second set of wheels or bike because you don't want to ride them all year long so it is not for everyone.

Again, if you'd just look at the example provided: the coldness really isn't the issue. I checked a few major Canadian cities, and Oulu was similar or colder, looked like, and it has plenty of biking going on.

Investment is the issue. Cities and countries largely aren't willing to invest anywhere close to the same money into bikes the same way they do cars, and then everyone feigns surprise that this somehow makes biking harder and less popular.