The possible duplicate that @Toto mentioned in his comment contains exhaustive information regarding the subject of prepositions and the case of cities, countries and the like. In brief

Développeur chez Apple à Cupertino

because Cupertino is a city. Furthermore

Je vais à Paris, à Boston, à Brest, à Grenonble,…

For masculine countries that their first letter is a consonant or for countries in plural (le Portugal, le Danemark, les États-Unis), since à+le–>au and à+les–>aux we use

au Portugal, au Danemark, aux États-Unis, etc.

For feminine countries and masculine one that start with a vowel we use en

en France (f), en Iran (m), en Espagne (f),…

For feminine regions or masculines that start with a vowel we use also en

en Normandie (f), en Provence (f), en Anjou (m)

For islands whose name contains article we use also en

en Corse (la Corse), en Sicile (la Sicile)…