How do I stop ‘apps running in the background’ on my iPhone X?

There is no need to actually stop apps. The official idea is that iOS takes care of it. Inactive apps are in stand-by, which is said to be better than force-quitting and re-launching.

You should force an app to close only when it’s unresponsive.

How to force an app to close on your iPhone, iPad, or iPod touch - Apple Support

There is also settings->general->background app refresh. From there you can disable all background apps altogether, or select just those you wish to execute in the background.