One UI is not a app so you can't open it but open the One Ui Home settings for customizations. In order to do so when you are in the app list in settings next to it you'll see like a settings icon tap on it and you'll be able to access it. If not you can do that from home screen. Long press on empty space on home screen and then you'll see options in the buttom, in there tap on Home Screen Settings, in there you can do requires customization. Also for customization and add more features in Android pie or One Ui you can download app made by Samsung from Galaxy Store which is called Good Lock.
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