Usually presenters will be mirroring what's on their Primary screen to the external screen. That is, what they see on their primary screen is exactly what the audience sees.
This is called video mirroring and you can set it up using these steps:
- Connect your second monitor and switch it on
- Go to System Preferences (under the Apple menu)
- Click on Displays
- Click the Arrangement tab.
- Tick Mirror Displays from the lower-left corner
If, in your case, you have a particular need to view something different on your Primary screen at the same time the audience is viewing the secondary screen, but you also want a preview of what's on the second screen displayed in a window on your Primary screen, then this is not possible within macOS itself.
However, I do believe I have seen specialised software used in churches, universities, etc that can do this. The specifics escape me at the moment, but if that's what you're after I may be able to find out.