What do you mean by "desktop" here exactly?
If you mean the black bar of stacked icons on the left, that is the launcher.
You can pin apps to the launcher by long pressing on them while they are running, and un-pin similarly. Running apps always appear there, and you can tap on the icons to switch.
The app spread is what you get with the long swipe from the right, showing the live previews of apps. This is basically the same as when you press Alt+Tab on a PC to switch between apps/windows.
Scopes are special and no you can't close the dash application, however in a future update the scopes will be going away completely. The replacement for the apps scope (where you currently launch applications), is the app drawer, which will be opened with long swipe from left, or by tapping the big logo button on the launcher.