As of this morning, California finally has a COVID-19 contact tracing app! I installed it this morning, and I’d encourage fellow California residents to do the same. If you, like me, use Android, install the CA Notify app from the Google Play store. (You may have even gotten a notification this morning encouraging you to do it.) If you have an iPhone, you don’t even need to install anything. Just go into Settings, click on “Exposure Notification,” and go from there.
If you want to understand how a contact tracing app helps us keep the virus under control, how this implementation (which uses the Google and Apple APIs) works, and why it’s taken so long to get built and adopted in the U.S., this New York Times article has a pretty good summary. If you’re worried about privacy, The Markup was on this early, and they also published an informative followup article in August.