How do I close all open apps? I’ve done what it instructs but doesn’t work n my phone won’t stay charged because too many apps open

Apps that are not the current, active app, are, with few exceptions, not using any significant amount of battery. Force closing them can, in fact, use more battery power than leaving them alone.

What will use power is if you have Background App Refresh enabled for apps. Go to Settings>General>Background App Refresh and turn them off.