I'm not sure ... how is your PC connected to Projector? is your PC configured with Projector as 2nd monitor?
Let's assume Projector is a simple 2nd monitor on computer (not network connected, or something else)
What will happen depends on how you have configured PowerPoint (or any other app). If an app opens up and displays on Monitor 2, then will be shown (be that full screen or a window, again per App setting, or Window position as retained by Operating System)
With more specifics, I or others can provide more specific feedback. But if a traditional display connection for Projector, this is an OS question, not OBS... I think {could easily be wrong}