It would basically pick up the signals of nearby phones at 5-minute intervals and store the connections between them in a database. If one person tests positive for the novel coronavirus, they could tell the app they’ve been infected, and it could notify other people whose phones passed within close range in the preceding days.
Apple and Google will introduce a pair of iOS and Android APIs in mid-May and make sure these health authorities’ apps can implement them. During this phase, users will still have to download an app to participate in contact-tracing, which could limit adoption. But in the months after the API is complete, the companies will work on building tracing functionality into the underlying operating system, as an option immediately available to everyone with an iOS or Android phone.