Patron Saint Herpes Zoster What Makes A Saint A Patron Saint Of A Country?

What makes a saint a Patron saint of a country? - patron saint herpes zoster

I wonder because I always thought that Joan of Arc, the patron saint of France, but Saint-Denis.

Is it more of a saint? What makes a saint, a patron saint of something?

I do not know many saints that I know and do not understand it.