Sorry, I don't follow your question / don't understand what you are trying to accomplish?
I'm assuming you are just getting started? if not, ignore this reply entirely
The primary video capture (a Source) methods are
- [Application] Window
https://obsproject.com/kb/window-capture-sources
- Display (ie, entire screen/monitor)
https://obsproject.com/kb/display-capture-sources
- Game capture
https://obsproject.com/kb/game-capture-source
the above from
https://obsproject.com/kb/sources-guide
That capture is a Source .. You can have many sources in a Scene (collection of Sources, Audio and Visual) in a OBS Studio 'scene' (a Scene=a Recording/Streaming output). You can then transition between many Scenes as you see fit (with same or different Sources, arranged differently).
Be careful with terminology as a 'screen' is typically consider the same as your monitor. Which has NOTHING to do with what you Record/Stream. Now, you can use the entire monitor as a Source... but that is simply one of many possible Sources to 'composite' into a given video output (Stream/Recording). A given OBS Studio Scene can contain multiple Window Captures.
With that said, I'm hoping you can provide some clarity on what you are trying to accomplish, because I can't tell from your post
