How can I stop by External Display from switching "Spaces" Somehow I enabled my mac to have these "spaces" and I always have a second "desktop" - I don't know how to get rid of it.

However, either when no apps are open or if there is a fullscreen app, I do not want my external thunderbolt monitor to switch to it's new "space". I only want to switch spaces on my laptop monitor. Is there a way to disable this? I can't even find out how to disable my second desktop, period.
I don't think exactly what you are asking for is possible, but this may be an acceptable solution: you can tell the application to show up in all spaces, and it will be as if your external display isn't switching spaces.

In order to do this, right click on the dock icon for your application, select "Options," and then "All Desktops"



Hope this helps!