Radon is denser, it gets sucked into your house because of the pressure difference. Since it is denser it will pool in the lower levels so the guidelines say to test the lowest floor where you spend a lot of time. If there’s not high amounts of radon in that location it won’t be in the upper floors.

Air circulation internally should cause the amount to average across your entire house. And internal to external ventilation is how you get it to dissipate. That’s why the guideline is to test for at least 3 months over the winter when houses are more sealed up. That way you get the worst case result and over the summer it would probably be less.

Radon is dangerous because it’s radioactive with a half life of 3 days which is a very human timescale compared to some other radioactive particles and if a radon particle happens to be inside your lungs when it does decay it will damage your lung tissue and long term repeated exposure will put you at greater risk of lung cancer.

I work for a health charity that does some work in the radon field. I won’t rehash what others have said other than it’s legit and relatively unknown. The only thing I’ll add is that modern home construction makes it worse since buildings are much more sealed than they used to be which causes the radon to build up instead of escaping into the surrounding atmosphere.

Some (maybe all at this point) provincial building codes now require a rough in pipe opening in your foundation so you can install a mitigation system if levels become a problem in your house.