Winter tires aren’t mandatory in Denmark, but of the 2.5M or so cars in Denmark, the ones going to Sweden from December to March are required to have them or the equivalent tread depth. Of the 5M or so cars in Sweden, a majority of which are registered in non-mountainous parts of Sweden, they are required to have winter tires or the equivalent in tread depth. And in Norway’s it’s more or less required to have tire arrangements suitable to the weather conditions on the driving terrain, so that’s going to mean doing something different with the tires based on the seasonal conditions.

In Japan, they don’t really plow and salt the roads as much as is done in the US. And even with winter tires in Japan, if you’re driving into mountainous terrain during winter, there may be times you’ll want chains even if they aren’t mandatory. And that’s even when you have winter tires. At some point in the future, don’t be surprised if Japan makes chains mandatory at times ... even for rental car use at times. Some Japanese rental car companies definitely do rent out snow chains for use on the rental cars.

It's getting more popular to have winter tyres in Denmark - especially if you are living close to Germany where the rule is similar to the Sweden. The law in Sweden stating that from Dec 1 all cars should have winter tyres when there's snow/ice/frost/slush on the road (source: https://www.transportstyrelsen.se/vinterdack) - with the weather condition now I don't know when we will change the tyres.

I'm not going to go into mountains in Japan, I'm wondering what kind of tyres people have on their car if they are living around Tokyo.

In Tokyo pretty much everyone just has summer tires because a) they are quieter and more fuel efficient, b) most people don't have space (very small houses/apartments) to store winter tires, c) snow rarely accumulates in the Tokyo area...maybe once every year or two, d) a lot of people use chains because they don't take up space and the areas where snow does fall tends to be mountainous and winter tires don't work that well on steep grades.