In Android, there's no need for an app to include a back or close button because there of the universal back button in the button bar at the bottom (and indeed Google/Android's Material design guidelines discourage it). This provides a uniform experience -- users know that they can use the universal back button in any app and expect a consistent behavior.
This was based on a UX design philosophy pioneered by Apple's introduction of standardized keyboard shortcuts like Command+C, Command+X, Command+V, and Command-Q for copy, cut, paste, and quit. All Apple software makers were encouraged to adopt the same shortcuts across all applications so that users didn't need to relearn the mappings for each software, as was customary before that time. If you are familiar with old text editors, like Emacs or Vim, you will be aware of some nonstandard shortcuts, like Control+Y for paste in Emacs, or P to paste in Vim.
Web apps, on the other hand, can't take advantage of the universal back button because the browser usually uses it for browser-back, and are tharefore more likely to have a close button on the side menu.
It's been a while since I've used an iPhone, but I remember back buttons within apps being more common on that system, even to the point where the same app will have a back button in the iPhone version but not in the Android version, because iPhones lack a standard back button. I believe that was done because Steve Jobs liked the visual aesthetic of having only one button without regard for functionality, leading to a hodgepodge of different visual designs from one app to another, an ironic reversal of their design philosophy from decades prior.