In order to say "This city is in France," we can say "Cette ville est en France." But, why is it that we say "Cette ville est aux Etats-Unis" for "This city is in the United States?" I don't exactly understand the difference between "en" and "aux."
Each country name has a gender and a number.
The country France is feminine singular, as demonstrated by the phrase "Vive la France!"
The country United States (Les États-Unis) is masculine plural.
For feminine country names, we use the preposition "en" while for masculine countries, we use "au" for masculine singular names, and "aux" for masculine plural names.
This explains why it is
"en France" and "aux États-Unis".
Other examples could be
"au Maroc", "au Canada", "en Grande-Brétagne", etc.