There are almost uncountable Android devices in the world—it is the world's largest computing platform, after all. Naturally, some apps in the Play Store have accumulated a lot of downloads. The billion-download club includes the likes of Google Calendar, Microsoft Word, and Snapchat. Now, you can add Samsung's home screen to the list.
All current Samsung phones come with the One UI home screen, and the Play Store listing allows Samsung to update devices with new features (it also uses its own app store in some countries). While the listing is currently labeled as "One UI," the Play Store page pre-dates Samsung's current interface and branding. The app has been accumulating downloads since 2012, so the "1 billion" milestone is not a measure of phones actually running One UI.
The latest version of the Samsung launcher includes features specifically for Android Pie including full-screen gesture navigation, locking the home layout, and more. This isn't the first Samsung app to hit 1 billion downloads. The Samsung Gallery did so a while back, as did the Samsung keyboard. Like the home screen, these come pre-loaded on a lot of phones.