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.
There is always a layer of crushed stone under the concrete slab, so the one port can pull air from underneath the entire slab. Air can always find a way through the concrete, the sump pump pit is a huge hole through the slab that also needs to be sealed.
A large portion of Canada is basically built on solid granite with little dirt. Our own house needed blasting through rock to dig the foundation.