For many states, such as California, you don't need to install any app. They use iOS's Exposure Notifications Express app framework, which can be activated from Settings | Exposure Notifications.
List of states that support Exposure Notification:
- Alabama (app)
- Arizona (app)
- California (express)
- Colorado (express)
- Connecticut (express)
- Delaware (app)
- District of Columbia (express)
- Hawaii (app)
- Louisiana (app)
- Maryland (express)
- Michigan (app)
- Minnesota (express)
- Nevada (express)
- New Jersey (app)
- New York (app)
- North Carolina (app)
- North Dakota (app)
- Pennsylvania (app)
- South Carolina (app)
- Virginia (app)
- Washington (express)
- Wisconsin (express)
- Wyoming (app)
Coming soon: