To build on what @Russ_Thomas has said, if you open a browser using WebUI.openBrowser(), then Katalon Studio will instantiate a WebDriver object and place it in a list with a static, thread-safe reference. Then, whenever that driver is needed for any action, it is retrieved from this list.
In the case that you are using a built-in WebUI keyword, such as (WebUI.navigateToUrl(), WebUI.click(), etc.), then that keyword will grab the driver instance for you behind the scenes.
In the case that you want to implement your own keyword, you can retrieve the instance that was created like so:
WebDriver driver = DriverFactory.getWebDriver()
Hopefully that answers your question.