3-1: if you're EU citizen in the US using only a US service then GDPR does not apply it falls under US data protection however if that US service is using an EU subprocessor then GDPR does apply
3-2: if you're a US citizen in EU you're under GDPR regardless of the service you're using
https://gdpr-info.eu/art-3-gdpr/
3-1: if you're EU citizen in the US using only a US service then GDPR does not apply it falls under US data protection however if that US service is using an EU subprocessor then GDPR does apply
3-2: if you're a US citizen in EU you're under GDPR regardless of the service you're using
Also https://gdpr-info.eu/recitals/no-23/ if you're a US company targeting EU residents you fall under GDPR