区分：WebElement, MobileElement, AndroidElement, and iosElement
Basically, using WebElement will let you use all the normal Selenium commands. MobileElement is appium's element which subclasses WebElement and adds appium-specific features (like being able to perform Touch Gestures).
AndroidElement and IOSElement implement MobileElement and add OS-specific features. Like on Android you can use findByUIAutomator and on iOS you can use findByUIAutomation