Google is pulling data from three sources: Johns Hopkins University, the New York Times, and Wikipedia. They receive their data from the World Health Organization and other public or government health organizations. Information is available for all 220 countries that Google Maps works in.
The feature will soon be available on iOS and Android versions of Google Maps. To use it, open the app, press the layers button (two squares on top of each other) on the top right and press “Covid-19 Info” button. That will reveal the color-coded app showing case counts.