MacOS full screen mode:

1) Getting into full-screen mode:

This varies for different apps.  A common method is CTL-CMD-F.
Others use FN-F. 

2) Getting out of full-screen mode:

This varies even more.  The "official" way is to just hit ESC,
but lots of apps ignore this or use it for something else. 
Those that use FN-F to get into full-screen often use it to get out, too.

If all else fails, CMD-OPT-CTL-ESC is supposed to kill the current app.
If that fails, but you can switch desktops or ssh in from another machine,
you might be able to kill the app that way.

Even if you don't ever use full-screen mode, it's probably worthwhile to
practice all of the above, so you might remember them when some app takes
over the screen without your permission (which is all too common).