How do I turn off my nintendo switch controller, without turning off my console itself?

Solution 1:

There is a small black button (the sync button on a Pro Controller) between the L/R buttons that switches off a controller as shown here

On the Joy Cons, it is in this location:

Arrow pointing to location of button on inner edge of Joy Con

In most games, switching off the controllers will pause the game until you switch them back on. I'm not sure if this applies to YouTube, though.