Moved into new (to us, built in 1980) house 2 years ago. Installed an Airthings radon meter a year ago. Levels get up to 400 Bq/m3 when all the windows are closed. Finally had a radon mitigation system installed. Will hopefully get to much safer level below 100 Bq/m3.
No, it happens pretty much everywhere.
No, it depends on the geology. Some granitic rocks, black shales, sedimentary rocks with high concentrations of uranium, and metamorphic rocks derived from the latter 3 will have high radon. Otherwise it’s not a problem.
Odds are your local or national government has published radon susceptibility maps with detailed info.
We can’t get radon in NW Florida. The largest natural rocks are usually pea gravel in creek beds or creek beaches. Only exception I’ve found after hiking and kayaking all over this corner of the state are some iron ore “boulders” at a city park, which is why they built a water mill there.
If you find a rock in the woods, it’s concrete or asphalt. :)
EDIT: If you go east a good bit there are limestone caves. Pretty neat, no radon.