My interpretation (which might be wrong) of the study abstract is that it is known that Vitamin D levels vary in different regions, and it's also known that COV

My interpretation (which might be wrong) of the study abstract is that it is known that Vitamin D levels vary in different regions, and it's also known that COVID-19 mortality varies. And these things are the motivation to study whether Vitamin D levels have a connection to mortality. So they measured Vitamin D levels and measured mortality and looked to see if there is a correlation.