I didn't realize you're on iPhone X. To pull up the most recently used apps, with any app open, swipe half way up the page and hold your finger there. This will bring up all the most recently used apps. Then to close any of them, put your finger on one of the apps and hold it there until you see a - sign on the top left corner. Either tap the - sign or swipe the up to close it.
Question: Overcharging of iphone X what will happen?