When do we use "en" or "à"?
This link covers it very well: https://www.frenchtoday.com/blog/french-grammar/french-prepositions-countries-cities-regions/
As EliasVanLo1 said, generally "à" is for cities and "en" is for countries. For states and regions, it seems to vary by the tradition of each location...so I think we have to learn them by heart. :-/