More than 8.1 million people in the U.S. have turned their iPhones and Android devices into pandemic contact-tracing tools, but it hasn't been of much use when their neighbors, classmates and coworkers aren't on the same system.
Apple and Google co-created “exposure notification" technology to alert phone users if they spent time near someone who tests positive for the coronavirus, so they know to get tested, too.
If you travel between the places where the technology is activated, in most cases it doesn't matter that you're using your home state's app. It still tracks if you've been exposed elsewhere. That's because a group of coordinating public health agencies has set up a shared national server of Bluetooth keys. A similar arrangement happens across some national borders in the European Union.
It was a collaborative project between Google, Apple, and various privacy advisors so any apps based that Google and Apple technology 'should' work across devices and apps.
See more about the alerting at https://www.seattlepi.com/news/article/EXPLAINER-How-phones-can-alert-you-to-COVID-19-15779707.php