Overcharging of iphone X what will happen?

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.

You phone can't overcharge. Once it's received a full charge the circuitry in the phone stops accepting a charge.

Go to Settings > Battery and see what's using the most power. Also make sure you've updated to the latest iOS 11.1.1

No, whether you charge via cable or wireless, it will NOT overcharge.

Make sure that app is fully off and not draining your battery.

What do you mean what do you do then? If you are asking how to fully close an App? Double tap your home button to bring up recently used apps and swipe up the offending app to fully close it.

Yes.