"To help ensure these anonymous tokens can't be used to identify you or your location, they change every 10-20 minutes. ... On a daily basis, your phone downloa

"To help ensure these anonymous tokens can't be used to identify you or your location, they change every 10-20 minutes. ... On a daily basis, your phone downloads a list of all the anonymous tokens associated with positive COVID-19 cases and checks them against the list of anonymous tokens it has encountered in the last 14 days." This page actually explains this pretty well for a non-tech-savvy audience.