Japan has a +4 years lead of life expectancy over Finland; Norway almost +3 years on Finland. I am not saying this is conclusive per se, but to me the sauna-people-live-forever is not backed up by the data. I would instead reason that, e. g. weight correlates a lot more here.

Your comparison reads to suggest that Japan doesn't have Onsen culture or that sauna does not exist in Norway.

That's to say, many cultures from around the globe have developed similar activities that heat the body.

Nobody is claiming they live forever. The claim is sauna use increases lifespan. There are other factors than just sauna use in lifespan though. The question is would the Japanese live even longer if they were using a sauna?

A lot of Mediterranean countries also have high life expectancy and are the opposite of a sauna culture.

Maybe eating a lot of fish, rather than meat, has an impact too.